Docker dotnet restore: Unable to load the service index for source https://api.nuget.org/v3/index.json

Objective -> I am currently trying to integrate Dapr with a small sample project provided by the .Net project templates. At the end of my test cases, I would like to use Docker Compose to launch multiple containers for a web API and a web app to communicate with each other using the Dapr sidecars.

Problem -> Whenever I run docker compose up on the sample application, the container fails to be created and is stopped by this error:

 > [build 4/7] RUN dotnet restore "MyBackEnd/MyBackEnd.csproj":
#0 0.945   Determining projects to restore...
#0 7.179 /src/MyBackEnd/MyBackEnd.csproj : error NU1301: Unable to load the service index for source https://api.nuget.org/v3/index.json.
#0 7.202   Failed to restore /src/MyBackEnd/MyBackEnd.csproj (in 6.09 sec).

Steps to Reproduce ->

The following are the steps I took with this project:

1. Ran mkdir sample_project && cd sample_project
2. Ran dotnet new webapi -o MyBackEnd followed by cd MyBackEnd
3. Ran touch Dockerfile && cd .. && touch docker-compose.yml

I then put this in my Dockerfile

FROM mcr.microsoft.com/dotnet/aspnet:6.0-focal AS base
WORKDIR /app
RUN apt-get update && apt-get install -y libcurl4
EXPOSE 80
EXPOSE 443

FROM mcr.microsoft.com/dotnet/sdk:6.0-focal AS build
WORKDIR /src
COPY ["MyBackEnd/MyBackEnd.csproj", "MyBackEnd/"]
RUN dotnet restore "MyBackEnd/MyBackEnd.csproj"
COPY . .
WORKDIR "/src/MyBackEnd"
RUN dotnet build "MyBackEnd.csproj" -c Release -o /app/build

FROM build AS publish
RUN dotnet publish "MyBackEnd.csproj" -c Release -o /app/publish

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "MyBackEnd.dll"]

And this was what I did with my Docker Compose YAML file:

version: '3.4'

services: 
  mybackend: 
    image: ${DOCKER_REGISTRY-}mybackend
    build: 
      context: .
      dockerfile: MyBackEnd/Dockerfile
    ports: 
      - "3000:80"

Finally, I ran docker compose up to arrive at the issue

Additional Information -> I am not running this on a Windows system. I am using Ubuntu 20, and thus, I do not have access to Visual Studio to handle the launching and configuration for me. I am aware that this is a heavily replicated question, but I have yet to see anything that specifically applies to users that do not have access to Visual Studio or on Ubuntu. Thanks!