Improve MW version extraction in Docker workflow

[tosfos]

The master branch workflow has two somewhat related issues related to the MW version extraction:

1. The Dockerfile uses FROM ghcr.io/canastawiki/canasta-base AS base (no fix version), which can cause failures when CanastaBase is updated. It's asking for trouble.
2. The workflow builds the entire Canasta image just to extract MW_CORE_VERSION from the environment variable. (See Remove hardcoding of mediawiki version #512.) This is an enormous and unnecessary waste.

To fix only the version extraction, we can use docker inspect.

To fix both issues, the easy way is to:

• Change the Dockerfile to pin CanastaBase to a particular version
• Extract the version from the pinned tag in Dockerfile using sed similar to how it was done pre-CanastaBase or use docker inspect

Or better and cleaner:

• Add a label in CanastaBase's Dockerfile to benefit anybody downstream
• Grab the label with docker inspect

[yaronkoren]

Oh, I had forgotten about the whole dummy build thing. I didn't know about "docker inspect", but that sounds like a good solution!

[vedmaka]

Indeed, adding a LABEL to the base image and retrieving it via something like https://github.com/regclient/actions would be a much better option than building the image on CI and retrieving the version from the built image

Although it feels like fixing the missing version on the FROM ghcr.io/canastawiki/canasta-base AS base is a much more important issue to fix. And adding a specific version to this instruction automatically solves the second problem of retriving the base image version

[yaronkoren]

Having a version number included in the CanastaBase image name sounds great also! It would enable maintaining working versions of Canasta (and other distributions) across different MediaWiki versions, something that's right now impossible.

[tosfos]

All done here