fix: add a CI job to test the container

[aryehb]

• Build the image from the Dockerfile, serve the site with it, and request a page to confirm the container actually works.
• Request the generated stylesheet as well as the home page.
  It is built by dart-sass and PostCSS inside the container, so serving it exercises the whole toolchain of the image rather than just Hugo.
• Poll for the server instead of sleeping a fixed amount, and bail out early if the container has already exited.
• Dump the container logs on failure to make a broken build diagnosable from the run alone.