cosalib/container_manifest.py: account for images that are repo:tag

[dustymabe]

enhance the garbage collection for the repo:tag we are trying to create a container image manifest for. In this case we were getting an error because someone had pulled the
quay.io/coreos-assembler/coreos-assembler:main image down from quay. It wasn't a manifest listed repo:tag so podman manifest exists wasn't returning true, but the create would fail.

The updated code here should handle this case.

This enhances the previous effort in 01e4e55.

[marmijo]

LGTM

[dustymabe]

oh fun.. CI trouble:

16:30:05  - jnlp -- terminated (2)
16:30:05  -----Logs-------------
16:30:05  2025/01/24 21:30:04 [go-init] No pre-start command defined, skip
16:30:05  2025/01/24 21:30:04 [go-init] Main command launched : /usr/local/bin/run-jnlp-client
16:30:05  panic: runtime error: invalid memory address or nil pointer dereference
16:30:05  [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4a6f52]
16:30:05  
16:30:05  goroutine 17 [running]:
16:30:05  main.run.func1()
16:30:05  	/go/src/github.com/openshift/jenkins/go-init/main.go:161 +0x92
16:30:05  created by main.run
16:30:05  	/go/src/github.com/openshift/jenkins/go-init/main.go:155 +0x22f