Use tag to store both current and target version

Hey, thanks for the tool!

With the current setup, one cannot see the current version of services it has set up, only the digest, for example:
```
# I don't see which version this is
caddy:alpine@sha256:5f5c8640aae01df9654968d946d8f1a56c497f1dd5c5cda4cf95ab7c14d58648
```

I was wondering whether we could misuse the tag to store both information, for example `caddy:alpine__v1.27.3`. The `--update` would then take the first part of the tag before the `__`, and update the version and the digest. For example `caddy:alpine__v1.2@sha256:XXX `-> `caddy:alpine__v1.3@sha256:YYY`

Trace the --update entry point and the existing Docker image tag and digest parsing, then inspect any tests covering update behavior. Done means preserving the base tag while storing the current and target versions as proposed, and updating both the tag and digest consistently.

Written by the indexing model from the issue text.