update opm dockerfile example to cache

[grokspawn]

Signed-off-by: Jordan Keister jordan@nimblewidget.com

Description of the change:
add cache build/use to example dockerfile, so folks don't run afoul of large FBCs' linear load times.

Motivation for the change:

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Docs updated or added to /docs
• Commit messages sensible and descriptive

[codecov]

Codecov Report

Merging #1033 (38b55cc) into master (614d6a9) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1033      +/-   ##
==========================================
+ Coverage   51.95%   51.96%   +0.01%     
==========================================
  Files         102      102              
  Lines        9215     9215              
==========================================
+ Hits         4788     4789       +1     
+ Misses       3514     3513       -1     
  Partials      913      913              

Impacted Files	Coverage Δ
alpha/veneer/semver/semver.go	60.50% <0.00%> (+0.50%)	⬆️

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[everettraven]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

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