Revert "docker: consider the tag when checking if a digest is up-to-date"

[kbukum1]

Reverts #13794

Since merging #13794, Docker updates have started failing with update_not_possible, causing Dependabot to close PRs when updates should have been possible.

Based on initial investigation, this may be related to missing safeguarding in the digest comparison logic—specifically, handling cases when the expected digest cannot be determined (for example, due to transient registry errors):

# If we can't determine the expected digest (e.g., due to transient registry errors),
# we can't prove the digest is out of date, so conservatively assume it's up to date.
# This prevents false positives where we incorrectly mark dependencies as needing updates.
next true if expected_digest.nil?

[yeikel]

I can confirm that reverting fixes the issue

updater | 2025/12/19 21:25:29 INFO Results:
updater | +------------------------------------------------------------------------------------------------------------------------------+
updater | | Changes to Dependabot Pull Requests |
updater | +---------+--------------------------------------------------------------------------------------------------------------------+
updater | | created | github/dependabot-update-job-proxy/dependabot-update-job-proxy ( from v2.0.20251124194534 to v2.0.20251219172147 ) |
updater | +---------+--------------------------------------------------------------------------------------------------------------------+
proxy | 2025/12/19 21:25:29 Skipping sending metrics because api endpoint is empty
proxy | 2025/12/19 21:25:29 9/35 calls cached (25%)

[yeikel]

I sent #13842 as a follow up for this

@kbukum1 Please take a look whenever you get a chance