Load from env should only happen if the value is unset.
Extract a buildEnvironment function and revert some changes to tests.
Signed-off-by: Daniel Nephin <dnephin@docker.com>
For an environment variable defined in the yaml without value,
the value needs to be propagated from the client, as in Docker Compose.
Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
