Building `master` or something like this was failing.

Newer versions (`master`) use things like `--platform=...`,
which are not supported unless we enable the new BuildKit building
backend.

This reverts commit 2a25b63bb6.

Looking at other roles, we trigger building regardless of this.
It's better to always trigger it, because it's less fragile.
If the build fails and we only trigger it on "git changes"
then we won't trigger it for a while. That's not good.

Triggering it each and every time may seem like a waste,
but it supposedly runs quickly due to Docker caching.

We used to store them in data/, but that seems inappropriate,
since it's just static configuration that the playbook can recreate.

Keeps up with a1f64f5159 (diff-0ccf69eb4d59a7645eb4d0a0b077e693948edb33ad06df043bba3fb30122879b)

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/716

This patch makes us use more fully-qualified container image names
(either prefixed with docker.io/ or with localhost/).

The latter happens when self-building is enabled.

We've recently had issues where if an image was removed manually
and the service was restarted (making `docker run` fetch it from Docker Hub, etc.),
we'd end up with a pulled image, even though we're aiming for a self-built one.
Re-running the playbook would then not do a rebuild, because:
- the image with that name already exists (even though it's something
else)
- we sometimes had conditional logic where we'd build only if the git
repo changed

By explicitly changing the name of the images (prefixing with localhost/),
we avoid such confusion and the possibility that we'd automatically pul something
which is not what we expect.

Also, I've removed that condition where building would happen on git
changes only. We now always build (unless an image with that name
already exists). We just force-build when the git repo changes.

The when statement is supposed to be on the block, not on the individual task.

It affects all tasks within the block (they're all to be executed when ma1sd is enabled and self-building is requested0.

The tag format used in the `ma1sd` repo have change. Versions no longer
start with 'v', and when building for non-amd64, we also need to strip
off the '-$arch' bit from the Docker image name.

Further, when building the .jar file, `ma1sd` currently names the .jar
based on the project's directory, which we call 'docker-src'. This means
other parts of the `ma1sd` build can't find the .jar file. Remedy this
by ensuring that the dir is called `docker-src/ma1sd`.

No migration steps should be required.

(adapting strings and URLs)