[docker] consolidate dockerfiles

[fdupress]

This allows us to parameterise later layers on earlier ones.

In practice, I want to use this to avoid rebuilding the base layer in PR and merge queue runs, building it only on main and release branches and tags. As a drive-by bonus, this means we can cache the base and build layers for reuse when building the test layer.

I've also added a hook to allow building from a local working copy. (By providing an EC_REPO argument when building the test target.) This should allow us to pre-build the test image once and for all for all post-build checks. (Using . as EC_REPO, perhaps—haven't really thought that far.)

[fdupress]

Converting to draft again, I think I want to also separate build-base as a layer. Doing so will require (ideally) identifying when we need to rebuild it due to EC dependency changes. But we can also let the full build layer install and update new EC deps in PRs and such.

To be discussed briefly.

[fdupress]

@strub this is now ready for review. It is only a refactor, which should not change any measurable behaviour (including on build time), but gives us the flexibility to change measurable behaviour later down the line. (In practice, a build of the test box, for example, can "shortcut" the build by specifying an intermediate base, base-build or build image to use as a build argument.)

See #1008 to see how I intend to use this to shortcut the PR builds.

[fdupress]

A happy and coincidental side effect of this is to make the build possible using podman. (Accidentally found this out by absent-mindedly building this on a machine that symlinks docker to run podman.)