Dockerfile: document build-args

[thaJeztah]

docker buildx build --quiet --call=outline .

BUILD ARG            VALUE                                         DESCRIPTION
GO_VERSION           1.25.7                                        sets the version of the golang base image to use.
BASE_DEBIAN_DISTRO   bookworm                                      sets the golang base image debian variant to use.
XX_VERSION           1.7.0                                         sets the version of the xx utility to use.
OSXCROSS_VERSION     11.3-r8-debian                                sets the MacOSX cross toolchain to use.
PACKAGE              github.com/docker/docker-credential-helpers   sets the package name to print in the "--version" output.

- What I did

- How I did it

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

FWIW; it's really annoying that it doesn't allow an empty line between; i.e., this doesn't work (doesn't show up in outline);

# PACKAGE sets the package name to print in the "--version" output.
#
# It sets the "github.com/docker/docker-credential-helpers/credentials.Package
# variable at compile time.
ARG PACKAGE=github.com/docker/docker-credential-helpers

Wondering if we should allow that; would that be possible? cc @crazy-max

[codecov-commenter]

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