Enable push workflow for PRs

[cimnine]

Related Issue:

New Behavior

The push workflow is also executed for PRs.

Contrast to Current Behavior

We PRs don't get automatic feedback on PRs, because the workflow is not triggered.

Discussion: Benefits and Drawbacks

Currently the workflow push was only called when someone pushed to this repository. But since only maintainers can push to this repository, PRs from outsiders were not triggering the pipeline.
This PR changes this, so that the pipeline is also triggered for PRs.

When implementing this change I feared that this may cause a problem, namely that this could be used to leak secrets we have configured for pushing to Docker Hub.

After consulting the documentation (and a chat with Github support because I was not certain I understood it the right way) I was assured that secrets should not leak due to someone meddling with the workflow files.

The documentation reads:

With the exception of GITHUB_TOKEN, secrets are not passed to the runner when a workflow is triggered from a forked repository.

Changes to the Wiki

n/a

Proposed Release Note Entry

n/a

Double Check

• I have read the comments and followed the PR template.
• I have explained my PR according to the information in the comments.
• My PR targets the develop branch.