Add support for DB password as a file. Add Support for backing up multiple DB's in one go. Simplified github workflow.

[pilotso11]

Add support for DB password as a file.

• New env var DB_PASS_FILE.

Add Support for backing up multiple DBs in one go.

• New env var DB_NAMES with a comma-separated list of databases to backup.

Simplified GitHub workflow.

• Removed the need for per-release subfolders with everything built from the template in the GitHub action.
• Add a GitHub action to push to Docker Hub with the appropriate semver.
• Add a smoketest action as well. (Probably you don't want this.)
• Added a test.sh to test the build locally (convenience - you may not want.)

Added PG 17

• Made pg17 the default latest.