.gitlab-ci.yml: push versioned tags to the container registry. (Closes:...

.gitlab-ci.yml: push versioned tags to the container registry. (Closes: !119)

Old container images except 161 and 162 were not preserved on
`registry.salsa.debian.org/reproducible-builds/diffoscope`:
https://salsa.debian.org/reproducible-builds/diffoscope/container_registry/218



Fix 328

Signed-off-by: Chris Lamb <lamby@debian.org>