CI: Build docker on PR and push when tags are created

[orgads]

No description provided.

[orgads]

You need to configure DOCKERHUB_USERNAME in vars and DOCKERHUB_TOKEN in secrets for push to work. The token should be generated for sipwise user (assuming you own it...).

[mika]

Thanks, basically LGTM, just wondering whether we should use the github docker registry instead? 🤔

(We - as in Sipwise - abandoned docker usage back in 2023-03 due to Docker's sunsetting of Free Team organizations, not claiming that this is a hard NO for us right now, but we're discussing/checking our options and what would serve useful)