ci: docker images now run against PRs

[damienwebdev]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/mage2docker/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[x] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Due to an old mistake, CI hasn't been building the docker images on pulls.

Fixes: N/A

What is the new behavior?

We now build docker images (but do not publish) on pulls.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information