Force jenkins user on exec commands

[diorcety]

replace #130
Allow to specify an different user for the run

docker.image('dummy').inside('-u root:root -e BUILDER_UID=1001 -e BUILDER_GID=1001 -e BUILDER_USER=jenkins -e BUILDER_GROUP=jenkins -e HOME=/home/jenkins) {
}

Will run
docker run -t -u 1001:1001 -u root:root ...
and
docker exec -u 1001:1001 ...

[andrewnicols]

How can this be used with DSL?
At the moment we do something like:

agent {
    label 'docker'
    image 'php:7.1'
}

Obviously we can specify extra args, but will these allow overriding the user in the run stage?

I guess one 'ideal' would be to take a user argument in the agent stanza.

[diorcety]

I rebased my PR on master

[romanek-adam]

Does this change break any existing usage? If so then it should be implemented conditionally, based on a new parameter etc.

[diorcety]

This is break only the usage when the user force the user as image parameters with -u root:root for example. The current behaviour seems to be an issue for many people currently

[diorcety]

Okay, I finally succeeded to test the behaviour.