One more question: if I setup pants.toml with:

[docker]
env_vars = ["GITHUB_SHA=development"]
build_args = ["GITHUB_SHA"]

[environments-preview.names]
docker_linux = "//:docker_linux"
local_mac = "//:local_mac"
local_linux = "//:local_linux"

And BUILD file:

local_environment(
    name="local_mac",
    compatible_platforms=["macos_arm64"],
    fallback_environment="local_linux",
)

local_environment(
    name="local_linux",
    compatible_platforms=["linux_x86_64"],
    fallback_environment="docker_linux",
)

docker_environment(
    name="docker_linux",
    platform="linux_x86_64",
    image="python:3.11.2-slim-bullseye",
    python_bootstrap_search_path=["/usr/local/bin"],
)

I get an error when I run

pants package ::

:

[ERROR] 1 Exception encountered:

Engine traceback:
  in `package` goal

DockerBuildContextError: Undefined value for build arg on the <redacted>:app target: The Docker environment variable 'GITHUB_SHA' is undefined. You may provide a value for this variable either in `[docker].env_vars` or in Pants's own environment.

If you did not intend to inherit the value for this build arg from the environment, provide a default value with the option `[docker].build_args` or in the `extra_build_args` field on the target definition. Alternatively, you may also provide a default value on the `ARG` instruction directly in the `Dockerfile`.

It looks like a bug

Yeah, looks like a bug, possibly the same as the other issue you mentioned

For this one https://github.com/pantsbuild/pants/issues/18544