Add GHA workflow to test container image builds

[eloquence]

Now that the we use a static site generator, we should ensure that merges to main don't break the builds - a simple build workflow as a required step should do the trick.

[SaptakS]

Just to add a note, with 11ty and netlify, you can also generate a preview (if that is something the team wants). That checks if it builds correctly and also gives you a preview which might make reviewing easier.