it’s the output type of the rule function that determines how it hooks into goals, right?

Yeah, more or less. For hooking into prebuilt rules, it’s based on: 1) Registering a

UnionRule

for your request type so that Pants knows to use your plugin. Unions are how we do polymorphism with the engine. 2) Creating a rule that takes your request type as input, and returns the uniform

BuiltPackage

-- Oh, good point on not creating an actual file…There’s a way to return an empty

BuiltPackage

, though, we can work around that easily if desired. Outside of that wonkiness with not creating a file, do you think using the

package

goal would make sense? Would you rather do

./pants package path/to/Dockerfile

or a new goal like

./pants docker path/to/Dockerfile

? Either are valid. --

how do i get access to a build directory?

Do you mean how do you read from pre-existing files? If so,

PathGlobs

, which is documented on the “file system” page If you instead mean a directory for where to run your process, whenever you run a

Process

, it runs in a chroot automatically, with only the input files you specified. This allows for hermetic builds.