podman build: --build-arg: does not fall back to environment

[edsantiago]

Difference between buildah bud and podman build: buildah defaults a build-arg to an envariable if it's set; podman does not:

$ export foo=bar
$ printf "FROM quay.io/libpod/alpine:latest\nARG foo\n" | ./bin/podman build --quiet --build-arg foo -
WARN[0000] missing "foo" build argument. Try adding "--build-arg foo=<VALUE>" to the command line
41187bb4760c7a6c46b3cf4c7ca2595c7432cf87c06c131fadb6fc9e509136c2

buildah, if it sees --build-arg foo without an explicit =something, will interpolate the environment variable $foo.

This may be a bug, or may be an intentional difference, or it may be a nothing-we-can-do-about-it difference. If it can't be fixed, perhaps it should be documented.

[rhatdan]

Give me a test for this one, as well, and I will give you a fix.

[edsantiago]

build-arg.diff.txt

Reconciling all these diffs is left as an exercise for the reader