Fix custom build example

[PriyaModali]

Fixes: #5492
Description
Make custom build example functional

Custom build example is currently broken. It needs to be fixed.

User facing changes
Before:

skaffold dev

Listing files to watch...
 - ko://github.com/GoogleContainerTools/skaffold/examples/custom
Generating tags...
 - ko://github.com/GoogleContainerTools/skaffold/examples/custom -> ko://github.com/GoogleContainerTools/skaffold/examples/custom:latest
Checking cache...
 - ko://github.com/GoogleContainerTools/skaffold/examples/custom: Error checking cache.
exiting dev mode because first build failed: getting imageID for ko://github.com/GoogleContainerTools/skaffold/examples/custom:latest: Error response from daemon: no such image: ko:/github.com/GoogleContainerTools/skaffold/examples/custom:latest: invalid reference format: repository name must be lowercase

After:

skaffold dev

 Listing files to watch...
 - github.com/googlecontainertools/skaffold/examples/custom
Generating tags...
 - github.com/googlecontainertools/skaffold/examples/custom -> github.com/googlecontainertools/skaffold/examples/custom:latest
Checking cache...
 - github.com/googlecontainertools/skaffold/examples/custom: Found Locally
Tags used in deployment:
 - github.com/googlecontainertools/skaffold/examples/custom -> github.com/googlecontainertools/skaffold/examples/custom:5ed1d68f2edf26a699842fbf3920fe964450e8bc714d4a2f4d0f307afa018bc7
Starting deploy...
 - pod/getting-started created
Waiting for deployments to stabilize...
Deployments stabilized in 49.503924ms
Press Ctrl+C to exit
Watching for changes...
[getting-started] Hello world!
[getting-started] Hello world!
[getting-started] Hello world!

[codecov]

Codecov Report

Merging #5495 (a5b9483) into master (2e55bec) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5495   +/-   ##
=======================================
  Coverage   71.42%   71.42%           
=======================================
  Files         398      398           
  Lines       14654    14654           
=======================================
  Hits        10467    10467           
  Misses       3409     3409           
  Partials      778      778           

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[MarlonGamez]

Looks good to me, tested locally and seems to work. I think we may have an issue with a code path not being hit. Left a comment on the linked issue

[briandealwis]

This isn't right: Skaffold has special treatment for ko: image references (#4952).

[PriyaModali]

This example was broken as the ko: / SanitizeImageName feature doesn't work properly now. Will revisit the example once the feature has been implemented properly.