Add image label info

[SvenDowideit]

I'd like to default some container / service labels using image labels - so that (for example) https://github.com/wemake-services/caddy-gen/ could have an image label virtual.port set in a Dockerfile label - so users don't need to set it on every container.

Similarly, exposing the port setting could be interesting - though more complicated to deal with when there are multiple ports set.

is this a bad idea? should I try to make a PR?