Adds a bats lint test that guards the static wiring (nginx conf, dokku-restore finish-script ordering, my_init sentinel reset, and the Dockerfile HEALTHCHECK line) plus a docker smoke test that boots the built image, waits for the health flip, exercises the loopback endpoint, asserts the port is not published to the host, and verifies the negative path. The smoke test is invoked via a new `make test-image-healthcheck` target and runs automatically in the build-image action after `docker buildx --load`.
Building/testing for ARM does not happen often - the only runtime environment is Raspberry PI, which supports ARM64 - and complicates support for a ton of features. Aside from that, CI runs are much longer for ARM Dokku images, often reaching 15-20 minutes or just timing out completely.
Rather than support an architecture that doesn't have much usage by maintainers and has a lot of maintenance burden, we're removing the platform.
