provide an optional base image flag for generated dockerfiles from alpha bundle build commands

[grokspawn]

Description of the change:
This PR adds an optional "base-image" flag to opm alpha bundle generate. This results in some breaking changes for possible clients which now must pass a new parameter "scratch" to produce previous behavior.

Motivation for the change:
Some clients prefer to use base images other than scratch. This is the lightest-touch approach I could use to provide this.

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

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: grokspawn

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• OWNERS [grokspawn]

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[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 46.80%. Comparing base (dc23843) to head (f6cf01e).
Report is 31 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/lib/bundle/build.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1519      +/-   ##
==========================================
- Coverage   48.61%   46.80%   -1.82%     
==========================================
  Files         135      135              
  Lines       12793    15760    +2967     
==========================================
+ Hits         6219     7376    +1157     
- Misses       5533     7343    +1810     
  Partials     1041     1041              

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[oceanc80]

/lgtm