Add lint warning for docker-push to GCR location not in team sub-directory

[robwhitby]

we want to promote a team based structure in the halfpipe-io GCR registry.

e.g. eu.gcr.io/halfpipe-io/my-image --> eu.gcr.io/halfpipe-io/my-team/my-image

a lint warning is a good first step to nudge people.

we can also update the docs and use the team sub-dir in examples.