ci: dont parallelize the docker image deps

[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?

Currently, builds accidentally pull the "latest" tag instead of using actually themselves. This is unintentional.

Fixes a bug uncovered in #70

What is the new behavior?

We no longer parallelize, but still keep the maintenance benefits of leveraging the matrix.

Does this PR introduce a breaking change?

[ ] Yes
[x] No