CI: fix the testing of docker images
CI: fix the testing of docker images

It was still using mvdan/shfmt-alpine:${TAG}, instead of
mvdan/shfmt:${TAG}-alpine. We never noticed because non-tag pushes would
succeed, since they'd pull an older image from the older repo.

The older repo is now deleted.

While at it, tweak the alpine docker image we use for testing.

mvdan/sh,mvdan/sh,mvdan/sh,mvdan/sh