Add checks for unset repository secrets

[felddy]

🗣 Description

This PR adds jobs to the build workflow that verifies that Docker and Foundry secrets are present.
If the secrets are not available, the jobs that require the secrets will be skipped, and informative
error messages are emitted.

💭 Motivation and Context

PRs from forks do not have access to repository secrets.
The CI UI did not make it obvious why the jobs were failing.
These extra jobs clearly throw errors into the annotations describing the problem.

🧪 Testing

Tested with GitHub actions by unsetting the FOUNDRY_USERNAME.

📷 Screenshots (if appropriate)

✅ Checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow project standards.
• All relevant repo and/or project documentation has been updated to reflect
  the changes in this PR.
• All new and existing tests pass.