Build docker test containers on release

[fdupress]

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[fdupress]

Quick note: I'd like to merge this before the 2026.03 release. But I'd also like to revisit this to try and reuse the fact that the base and build boxes could be relied upon instead of being rebuilt by the test job. If time permits, before merging; otherwise, later.

[fdupress]

The container is tested and functional. (Tested on WSL docker, and on a standard Linux via docker and podman.)

[fdupress]

This is properly ready to go now.

• Building the test box doesn't rebuild the whole stack of boxes;
• The test box is not built on the main branch; and
• The exclusion condition has been tested.

In addition, I've streamlined the jobs a little. This could be done as a matrix forced to run sequentially (not sure how I'd do the exclusion for the test box, then), but this feels simple enough to maintain.

@strub --- quick review ahead of the release? (REVERT ME will be reverted before merge; it's only there to test the workflow and its exclusion condition before merge)