feat: not enforce KO_DOCKER_REPO with push=false

[dirien]

Hi everyone,

This PR fixes #590 and was good for me to get used with the codebase of ko.

[codecov-commenter]

Codecov Report

Merging #604 (21e76c3) into main (eb2597f) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #604   +/-   ##
=======================================
  Coverage   48.51%   48.51%           
=======================================
  Files          43       43           
  Lines        2259     2259           
=======================================
  Hits         1096     1096           
  Misses        978      978           
  Partials      185      185           

Impacted Files	Coverage Δ
pkg/commands/resolver.go	30.41% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update eb2597f...21e76c3. Read the comment docs.

[imjasonh]

Nice! Thanks for this contribution!