Internal Registry - Add support to choose from custom service account

[weizenberg]

Hey there!

It would be great if you can add support for custom ServiceAccount. The current default service account is builder:

obu/pkg/cmd/cli/cmd/internal_registry.go

Line 76 in 63751a6

saName := "builder"

I'm using a custom build agent but for use your lib I need to change it to the default one (builder).