fix: update the Dockerfile

[aryehb]

• Fix casing for AS.
  The mismatch between FROM and as triggered a warning on every build.
• Fix CMD.
  • Each arg needs to be a separate array item.
  • Use the serve alias to match the non-Docker instructions in the README.
  • Bind hugo to 0.0.0.0 so the port can be forwarded to the host.
• Update the Hugo download command.
  • Add error handling (-S, test).
  • Update the regex to be more specific.
    • Hugo has 2 variants that match hugo_extended: hugo_extended and hugo_extended_withdeploy. Update the regex to exclude the unwanted one.
    • Escape the dots and add anchors.
  • Don't use the version variable to name the assets.
      This is a temporary container, so there's no need for the version in the filename.
• Fix downloading Sass.
  • Add error handling.
  • Update the regex to be more specific.
  • Don't use the version variable to name the assets.
      This is a temporary container, so there's no need for the version in the filename.
  • Move the entire dart-sass folder.
      sass is just a shell script that invokes the actual binary. The binary needs to be copied as well.
• Update the README with instruction for Docker.

[aryehb]

Tested locally, working properly.

[aryehb]

The next step would be to add a CI job that built the container and checked that it's serving content. This would add a regression test to ensure that the Dockerfile works.

[airween]

Thanks for this addition, @aryehb!