Why is Guard not detecting file changes after dockerizing Rails app?

I am trying to "dockerize" an existing Rails development app. This is my first time experimenting with Docker.

I want to set up Guard to listen for file changes and run relevant specs.

The Guard service appears to be running correctly, and the logs show:

guard_1     | 16:35:12 - INFO - Guard is now watching at '/app'

But when I edit/save spec files Guard is not running any tests.

This is an existing app that I'm moving into Docker. It has a guardfile that works outside of Docker.

I've searched and read a number of posts (e.g. this one), but I'm not sure where to start debugging this. Can anyone point me in the right direction and get Guard listening to file changes.

My docker-compose.yml looks like this:

version: '3'

services:
  postgres:
    ports:
      - "5432:5432"
    volumes:
      - $HOME/postgres-data:/var/lib/postgresql
    image: postgres:9.6.9

  redis:
    ports:
      - "6379:6379"
    depends_on:
      - postgres
    image: redis:5.0-rc

  web:
    build: .
    ports:
      - "3000:3000"
    command: /bin/sh -c "rails s -b 0.0.0.0 -p 3000"
    depends_on:
      - postgres
      - redis
    env_file:
      - .env

  guard:
    build: .
    env_file:
      - .env
    command: bundle exec guard --no-bundler-warning --no-interactions

  sidekiq:
    build: .
    command: bundle exec sidekiq -C config/sidekiq.yml
    depends_on:
      - postgres
      - redis
    env_file:
      - .env

volumes:
  redis:
  postgres:
  sidekiq:
  guard:

Guardfile

guard 'spring', bundler: true do
  watch('Gemfile.lock')
  watch(%r{^config/})
  watch(%r{^spec/(support|factories)/})
  watch(%r{^spec/factory.rb})
end

guard :rspec, cmd: "bundle exec rspec" do
  require "guard/rspec/dsl"
  dsl = Guard::RSpec::Dsl.new(self)

  # RSpec files
  rspec = dsl.rspec
  watch(rspec.spec_files)

  # Ruby files
  ruby = dsl.ruby
  dsl.watch_spec_files_for(ruby.lib_files)

  # Rails files
  rails = dsl.rails(view_extensions: %w(erb haml slim))
  dsl.watch_spec_files_for(rails.app_files)
  dsl.watch_spec_files_for(rails.views)

  watch(rails.controllers) do |m|
    [
      rspec.spec.call("routing/#{m[1]}_routing"),
      rspec.spec.call("controllers/#{m[1]}_controller"),
      rspec.spec.call("acceptance/#{m[1]}")
    ]
  end

  # Rails config changes
  watch(rails.spec_helper)     { rspec.spec_dir }
  watch(rails.routes)          { "#{rspec.spec_dir}/routing" }
  watch(rails.app_controller)  { "#{rspec.spec_dir}/controllers" }

  # Capybara features specs
  watch(rails.view_dirs)     { |m| rspec.spec.call("features/#{m[1]}") }
  watch(rails.layouts)       { |m| rspec.spec.call("features/#{m[1]}") }

  # Turnip features and steps
  watch(%r{^spec/acceptance/(.+)\.feature$})
  watch(%r{^spec/acceptance/steps/(.+)_steps\.rb$}) do |m|
    Dir[File.join("**/#{m[1]}.feature")][0] || "spec/acceptance"
  end
  ignore %r{^spec/support/concerns/}
end

guard 'brakeman', :run_on_start => true do
  watch(%r{^app/.+\.(erb|haml|rhtml|rb)$})
  watch(%r{^config/.+\.rb$})
  watch(%r{^lib/.+\.rb$})
  watch('Gemfile')
end