save: reject docker transport image references

[Rajkaran-122]

Summary

• The change to podman save.
• Rejection of docker:// image references with a clearer error.
• Addition of the e2e test.
• Referenced issues using: Non-actionable error message: 'Error: unsupported transport "docker" for looking up local images' #20775.

Checklist

• Certify you wrote the patch or otherwise have the right to pass it on as an open-source patch by signing all commits (git commit -s).
• Referenced issues using Fixes: #00000 in commit message (if applicable)
• Tests have been added/updated (or no tests are needed)
• Documentation has been updated
• All commits pass make validatepr
• Release note entered below (or no user-facing changes)

Does this PR introduce a user-facing change?

No

When `podman save` is invoked with `docker://{image}`, a more appropriate error is now returned.

[TomSweeneyRedHat]

LGTM
@Rajkaran-122 TYVM. If you're feeling energetic, similar changes could most likely occur for rmi, inspect, and maybe other commands.

[Rajkaran-122]

LGTM @Rajkaran-122 TYVM. If you're feeling energetic, similar changes could most likely occur for rmi, inspect, and maybe other commands.

Thank you for the review and the helpful suggestion! I'll investigate whether similar validation can be applied to rmi, inspect, and other commands.

[Rajkaran-122]

Thanks again @TomSweeneyRedHat sir for the suggestion. I opened follow-up PRs for rmi and inspect implementing similar validation:

#29332 (rmi)
#29334 (inspect)

[Luap99]

#29334 (review)

same as on the other PRs

[Rajkaran-122]

Thanks for the review @Luap99 @TomSweeneyRedHat . Following the feedback, I moved the discussion to the shared backend implementation in podman-container-tools/container-libs#1044 instead of continuing with command-specific validation here. I'll update this PR based on the outcome there.