Set default image, tags, before_script, and cache in .gitlab-ci.yml

Why is this change being made?

This is part of gitlab-org/gitlab#212970 (closed).

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what