Generate docker config in temp dir during image signing

[tomkennedy513]

No description provided.

[codecov-commenter]

Codecov Report

Merging #1157 (61baf85) into release/v0.7.x (cf95c76) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##           release/v0.7.x    #1157      +/-   ##
==================================================
- Coverage           70.58%   70.56%   -0.03%     
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  Files                 121      121              
  Lines                5729     5724       -5     
==================================================
- Hits                 4044     4039       -5     
  Misses               1311     1311              
  Partials              374      374              

Impacted Files	Coverage Δ
pkg/apis/build/v1alpha2/build_pod.go	98.65% <ø> (-0.01%)	⬇️

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[chenbh]

If i'm understanding this correctly, this is to deal with platforms cough openshift cough where security policies are in place that forces every pod to run under a random uid/gid. In such platforms, we can't chmod the home directory and we can't explicitly configure the running user, so any attempts to write the .dockerconfig.json will fail.

So the solution here is to generate a tempdir and set $DOCKER_CONFIG env var (which supported in ggcr)