Ensure we can access layer by diffID on a mutated image

[natalieparellano]

Previously, calling LayerByDiffID() immediately after adding a layer would error with the layer not found.

By ensuring that we call "compute" on the image before trying to access the diffID map, we ensure that the layer is found.

[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.65%. Comparing base (8dadbe7) to head (220652e).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/v1/mutate/image.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1875      +/-   ##
==========================================
- Coverage   71.67%   71.65%   -0.03%     
==========================================
  Files         123      123              
  Lines        9935     9938       +3     
==========================================
  Hits         7121     7121              
- Misses       2115     2117       +2     
- Partials      699      700       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[natalieparellano]

@codecov-commenter I disagree! Maintainers, any opposition to merging this one?

[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.