Replace docker/login-action with inline docker login

[inahga]

Drop dependency on docker/login-action by replacing it with a simple docker login command. Just reduces our supply chain exposure a bit.

This is done in a way consistent with GitHub's suggestions https://docs.github.com/en/actions/how-tos/write-workflows/choose-what-workflows-do/use-secrets#using-secrets-in-a-workflow, in particular: the secret is passed to the inner step as an environment variable; printenv prevents the secret from being visible in the process list, and is piped over stdin to docker login.

[inahga]

We lose logout at the end of the job, but I think this is moot. The runner is discarded after each matrix run.

[aarongable]

LGTM, with the caveat that I'd like to see some sort of indication (e.g. in the PR description) that this is an accepted best-practice way to do this. GitHub Action supply chain dependency attacks are very real. They also protect us from very real accidental errors, like getting the difference between "$DOCKER_PASSWORD" and ${DOCKER_PASSWORD} wrong.

[inahga]

Note that zizmor flags both the main branch and this PR for:

warning[secrets-outside-env]: secrets referenced without a dedicated environment
  --> .github/workflows/boulder-ci.yml:84:25
   |
28 |   b:
   |   - this job
...
84 |           username: ${{ secrets.DOCKER_USERNAME}}
   |                         ^^^^^^^^^^^^^^^^^^^^^^^ secret is accessed outside of a dedicated environment
   |
   = note: audit confidence → High

My interpretation of this and https://docs.zizmor.sh/audits/#secrets-outside-env, is that it is currently benign. The belt-and-suspenders fix would be to scope the token to an environment and configure environment protection rules. But since the token is still not available to forks, I think we're fine.