Is there a way to set a default value for docker build arguments with fallback to environment variables, e.g. for CI?

docker_image(
    name="my-service",
    registries=["@my_registry"],
    image_tags=["{build_args.EXPECTED_TAG}"]
)

When I build this image locally I would prefer to fallback to

latest

, e.g. only type

pants package /path/to/service/Dockerfile

but in CI I might provide it as

EXPECTED_TAG=1.0.2 pants package /path/to/service/Dockerfile

. I've tried setting it in the docker_env_vars, but subsequently exposing it in the shell does not seem to have precedence over this. If I don't provide the EXPECTED_TAG the build errors out about missing variables.

You can use

.pants.rc

https://www.pantsbuild.org/docs/options#pantsrc-file

you can also provide the builld arg with a value in your

Dockerfile

that way you have a very low prio default value that pants will replace with anything else you provide.

I didn't think about using

.pants.rc

. I'm not sure it fits the bill here since it can't be overridden using a command line env-var, but it could have other uses, so I appreciate the suggestion. I think you're right @curved-television-6568. This does serve as a nice fallback that can be easily overridden on the command line!