Ensure that container images are only pushed when running on default branch

[gvegayon]

FYI, this action will never pass on this PR because publishing to the container registry only happens on merge into main.

Ping @gvegayon

Alright, but we need to address that later (not today). We can add a conditional if for pushing the container only if running on the default branch. Approving for now!

Originally posted by @gvegayon in #28 (review)

[olivia-banks]

Should be solved in #31.