[JENKINS-51397] Pass environment to docker login command

[jglick]

Picks up jenkinsci/docker-commons-plugin#71.

[jglick]

I should point out that I am not sure how to reproduce the reported bug manually, much less write an automated test for it (which would perhaps involve Docker-in-Docker, very problematic). I did run some experiments where I created a fake Docker installation which just printed its environment and arguments, and ran a representative program with this (a variant of the script I originally used to verify that the fix of JENKINS-38018 allowed me to log in to DockerHub). Before this patch, the environment was just that of the agent generally plus DOCKER_CONFIG; after the patch, it included environment variables specific to the build and step context, as it would be for example for a sh step at the same point.