How do I get a background image with gradient within a container that has margin on it?

:root {
  --menu-width: 240px;
}

.nav {
  position: fixed;
  top: 0;
  bottom: 0;
  left: 0;
  z-index: 10;
  width: var(--menu-width);
  background-color: #ff0099;
}

.main {
  position: relative;
  margin-left: var(--menu-width);
  min-height: 100vh;
}

.main-background {
  content: ''; 
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: var(--menu-width);
  background-image: url(https://images.unsplash.com/photo-1477233534935-f5e6fe7c1159?ixlib=rb-1.2.1&ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&auto=format&fit=crop&w=1740&q=80);
  background-repeat: no-repeat;
  background-size: 100% auto;
}

.main::before,
.main::after {
  position: absolute;
  z-index: 1;
  right: 0;
  left: 0;
  content: '';
  background-image: linear-gradient(180deg, rgba(2,0,36,0) 0%, rgba(0,0,0,1) 100%);
}

.main::before {
  top: 0;
  height: 100vh;
  background-image: linear-gradient(180deg, rgba(2,0,36,0) 0%, rgba(0,0,0,1) 100%);
}

.main::after {
  content: '';
  top: 100vh;
  bottom: 0;
  background-color: #000000;
}

.container {
  position: relative;
  z-index: 2;
  margin: 0 auto;
  padding: 1rem 2rem;
  padding-top: 24rem;
  max-width: 960px;
  color: white;
}

<link href="https://cdnjs.cloudflare.com/ajax/libs/normalize/8.0.1/normalize.min.css" rel="stylesheet"/>
<div>
  <nav class="nav"></nav>
  <main class="main">
    <div class="main-background"></div>
    <div class="container">
      <h1>This is a title</h1>
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      <p>Sea of Tranquility billions upon billions emerged into consciousness explorations inconspicuous motes of rock and gas stirred by starlight. Network of wormholes the sky calls to us a mote of dust suspended in a sunbeam the carbon in our apple pies take root and flourish kindling the energy hidden in matter. Shores of the cosmic ocean from which we spring extraordinary claims require extraordinary evidence dispassionate extraterrestrial observer a very small stage in a vast cosmic arena not a sunrise but a galaxyrise.</p>
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      <p>Extraordinary claims require extraordinary evidence as a patch of light galaxies Rig Veda Cambrian explosion the carbon in our apple pies. Stirred by starlight hydrogen atoms hydrogen atoms gathered by gravity dispassionate extraterrestrial observer the only home we've ever known. Two ghostly white figures in coveralls and helmets are softly dancing descended from astronomers made in the interiors of collapsing stars Tunguska event the only home we've ever known brain is the seed of intelligence.</p>
      <p>Quasar explorations decipherment star stuff harvesting star light the ash of stellar alchemy from which we spring? A mote of dust suspended in a sunbeam the only home we've ever known courage of our questions dispassionate extraterrestrial observer take root and flourish realm of the galaxies? Citizens of distant epochs a very small stage in a vast cosmic arena Sea of Tranquility the only home we've ever known white dwarf vastness is bearable only through love.</p>
      <p>Rogue dream of the mind's eye cosmic ocean two ghostly white figures in coveralls and helmets are softly dancing hundreds of thousands Orion's sword? Paroxysm of global death bits of moving fluff descended from astronomers the ash of stellar alchemy Vangelis as a patch of light? At the edge of forever made in the interiors of collapsing stars the sky calls to us paroxysm of global death at the edge of forever the only home we've ever known?</p>
      <p>Hydrogen atoms invent the universe Drake Equation two ghostly white figures in coveralls and helmets are softly dancing Rig Veda a billion trillion. Stirred by starlight courage of our questions extraordinary claims require extraordinary evidence the ash of stellar alchemy kindling the energy hidden in matter courage of our questions. Rich in heavy atoms bits of moving fluff a mote of dust suspended in a sunbeam courage of our questions emerged into consciousness decipherment.</p>
      <p>Globular star cluster Tunguska event the ash of stellar alchemy the only home we've ever known rich in mystery great turbulent clouds? Not a sunrise but a galaxyrise at the edge of forever white dwarf intelligent beings courage of our questions muse about. Bits of moving fluff with pretty stories for which there's little good evidence stirred by starlight a still more glorious dawn awaits take root and flourish star stuff harvesting star light.</p>
      <p>Hearts of the stars rings of Uranus tendrils of gossamer clouds network of wormholes another world as a patch of light? Muse about globular star cluster a still more glorious dawn awaits Rig Veda cosmic ocean shores of the cosmic ocean. Rig Veda descended from astronomers concept of the number one Sea of Tranquility emerged into consciousness Rig Veda. A mote of dust suspended in a sunbeam gathered by gravity two ghostly white figures in coveralls and helmets are softly dancing the carbon in our apple pies extraordinary claims require extraordinary evidence invent the universe?</p>
      <p>Tunguska event made in the interiors of collapsing stars quasar Flatland not a sunrise but a galaxyrise hearts of the stars. Great turbulent clouds the sky calls to us tingling of the spine a mote of dust suspended in a sunbeam extraordinary claims require extraordinary evidence extraordinary claims require extraordinary evidence. Vastness is bearable only through love two ghostly white figures in coveralls and helmets are softly dancing permanence of the stars courage of our questions dream of the mind's eye a very small stage in a vast cosmic arena.</p>
      <p>Encyclopaedia galactica decipherment the sky calls to us billions upon billions the only home we've ever known hundreds of thousands. Euclid made in the interiors of collapsing stars bits of moving fluff from which we spring realm of the galaxies with pretty stories for which there's little good evidence. Dispassionate extraterrestrial observer network of wormholes take root and flourish dispassionate extraterrestrial observer with pretty stories for which there's little good evidence not a sunrise but a galaxyrise?</p>
      <p>Rig Veda of brilliant syntheses hearts of the stars the only home we've ever known Sea of Tranquility extraplanetary. Take root and flourish Drake Equation cosmic ocean invent the universe rings of Uranus network of wormholes. Not a sunrise but a galaxyrise Drake Equation laws of physics not a sunrise but a galaxyrise two ghostly white figures in coveralls and helmets are softly dancing a mote of dust suspended in a sunbeam.</p>
      <p>Decipherment Hypatia astonishment great turbulent clouds at the edge of forever Rig Veda. Muse about as a patch of light vanquish the impossible hundreds of thousands across the centuries ship of the imagination. Not a sunrise but a galaxyrise take root and flourish gathered by gravity shores of the cosmic ocean courage of our questions vastness is bearable only through love? Invent the universe the only home we've ever known courage of our questions kindling the energy hidden in matter the carbon in our apple pies invent the universe.</p>
      <p>Hypatia vanquish the impossible star stuff harvesting star light permanence of the stars rings of Uranus Sea of Tranquility. Rich in heavy atoms emerged into consciousness are creatures of the cosmos muse about the ash of stellar alchemy a very small stage in a vast cosmic arena. With pretty stories for which there's little good evidence encyclopaedia galactica inconspicuous motes of rock and gas two ghostly white figures in coveralls and helmets are softly dancing dream of the mind's eye citizens of distant epochs.</p>
      <p>Consciousness Sea of Tranquility citizens of distant epochs Vangelis science shores of the cosmic ocean. Venture another world extraordinary claims require extraordinary evidence are creatures of the cosmos concept of the number one the only home we've ever known. A very small stage in a vast cosmic arena the sky calls to us gathered by gravity hearts of the stars hundreds of thousands rich in heavy atoms? A mote of dust suspended in a sunbeam rich in heavy atoms not a sunrise but a galaxyrise two ghostly white figures in coveralls and helmets are softly dancing the only home we've ever known courage of our questions.</p>
      <p>Worldlets a mote of dust suspended in a sunbeam hundreds of thousands intelligent beings Cambrian explosion trillion? Brain is the seed of intelligence courage of our questions citizens of distant epochs emerged into consciousness the ash of stellar alchemy corpus callosum. Rich in heavy atoms from which we spring a still more glorious dawn awaits extraordinary claims require extraordinary evidence across the centuries the carbon in our apple pies?</p>
      <p>Tesseract billions upon billions vanquish the impossible dispassionate extraterrestrial observer a mote of dust suspended in a sunbeam dream of the mind's eye. Descended from astronomers ship of the imagination inconspicuous motes of rock and gas made in the interiors of collapsing stars the carbon in our apple pies the ash of stellar alchemy. From which we spring muse about star stuff harvesting star light stirred by starlight permanence of the stars rings of Uranus and billions upon billions upon billions upon billions upon billions upon billions upon billions.</p>
      <p>
    </div>
  </main>
</div>