fix(docker): slim down jobs, ensure auth happens

[malept]

What this PR does / why we need it

• Reduce the amount of setup in the shared/docker_* jobs by replacing setup_environment with the bare minimum setup required to authenticate to Docker image registries (via the shared/docker_auth command). This saves about 1m30s in every (successful) CircleCI run. A side effect of this was to avoid running the mage version command, which requires installing Go.
• Use both the push and pull registry values from either the environment or the box config to determine which registries need to be authenticated.
• Make sure that the ECR repository exists before pushing to it.

Jira ID

DT-4462

[getoutreach-ci-2]

Link to code coverage report (posted by coverbot 🤖)

[getoutreach-ci-2]

🎉 This PR is included in version 2.29.0-rc.6 🎉

The release is available on:

• GitHub release
• CircleCI Orb

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