Test container build workflows without push step

[glemieux]

Due to organization security settings, it is not currently possible (or recommended) to push new container images to the org's github package registry. To avoid confusion, it'd be best to find a way to avoid the push step when testing the workflow. The error will look like this otherwise:

#18 pushing layers
#18 ...

#19 [auth] ngeet/containers/spack-env:pull,push token for ghcr.io
#19 DONE 0.0s

#18 exporting to image
#18 pushing layers 0.7s done
#18 ERROR: failed to push ghcr.io/ngeet/containers/spack-env:latest: denied: installation not allowed to Write organization package
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 > exporting to image:
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 1 warning found (use docker --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 2)
ERROR: failed to build: failed to solve: failed to push ghcr.io/ngeet/containers/spack-env:latest: denied: installation not allowed to Write organization package
Reference
Check build summary support
Error: buildx failed with: ERROR: failed to build: failed to solve: failed to push ghcr.io/ngeet/containers/spack-env:latest: denied: installation not allowed to Write organization package