Overhaul the container and workflow

[nogweii]

This is a huge change to the process around how the containers are made.

Any push to the main branch, or whenever the workflow is triggered remotely (which is done by these workflows: stable and preview) the following the happen:

1. Set up the build environment, get docker installed, etc.
2. Fetch release information about from Github's API, against each of the repos
3. Build and push container images, but only the ubuntu base
4. Then send a message to Discord about the new image (with slightly different text / color for stable vs preview)

The container is also overhauled:

• There is only 1 Dockerfile now
• It requires environment variables to be set in order to configure it
• But sensible defaults are defined
• It only ever logs to stdout, not to a file.

Open question:

There is a container testing step with how the old workflow ran. Do we want to keep that? It's currently commented out, and would require a little more tweaking to the workflow to handle it but it's not super difficult.

[Syer10]

It all looks good to me. I think we should try to keep the test step since it has caught a few issues with preview before, allowing us to know there was an issue before people started posting bug reports.