Ability to use default working directory within a container

[sponte]

This is useful when you have you build your own container and want to execute
a process that produces the output and you don't necessarily want to
operate on files from Jenkins' workspace. Our usecase is that we pre-build
our application tests as a container and want to execute mvn inside of it.
We don't want to have to specify pom.xml path in the existing mvn command.

[sponte]

Hello @jonhermansen,

Sorry to bug you directly but you seem to be quite active in this repository and you merged a few changes in the past. Would you have time to look at this MR and comment/merge for us? I would like to avoid creating a duplicate plugin just for this small change.

Thanks in advance,
Stan