Ergonomic, hermetic, user-friendly software build system for Python, Go, Java, Kotlin, Protobuf, Scala, Shell. Pants lets you fearlessly scale up your codebase!

Pants

I can get round the issue by using 
```
dockerfile(
    sources=globs('Dockerfile')
)
```
but I’m hoping to get some understanding on how to set up defaults correctly

Not sure, pattern-matching here alone, but in your gist you construct a raw Payload instead of taking the one passed in to the constructor, if any. This goes against other examples.

Also, although I'm sure it's not related, watch out for the OSX HFSism of head. Should be HEAD but of course you can get away with it on standard OSX setups.