Ensure image tag doesn't drift from protocol versions #623

The quickstart image Stellar provides continues to update if you're always pulling @latest. Some recent significant protocol changes make this play poorly with mixing CLI versions on one protocol and the image on a different one.

#614 hard codes the image tag to use in the CLI, pinned to an explicit image related to protocol 27.

We should have some kind of guard up to prevent this hardcoding from becoming a foot-gun later. I might happen related to GH actions checks, or we could add a lookup table to track protocol version/image combos.