Fix krane:debug image to include shell

[imjasonh]

KO_CONFIG_PATH=./.ko/debug was being interpreted as relative to cmd/krane when we cd'd into it. Making it an absolute path fixes this.

Fixes #1262

Tested locally and produced gcr.io/imjasonh/krane:debug:

$ docker run --rm --entrypoint sh gcr.io/imjasonh/krane:debug -c "ls /"
bin
boot
busybox
...

[codecov-commenter]

Codecov Report

Merging #1264 (17ae660) into main (03d4ad1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1264   +/-   ##
=======================================
  Coverage   73.77%   73.77%           
=======================================
  Files         111      111           
  Lines        8286     8286           
=======================================
  Hits         6113     6113           
  Misses       1571     1571           
  Partials      602      602           

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