Add ability to set network for Docker Custom Build Environment plugin

[guetta14]

This PR is for adding the ability to configure the network parameter of docker when running inside a container.
When configuring the "build inside docker" section of a job - under advanced options - you have the ability to configure manually the network and change it from "bridge" (the default) to "host"
in the job-dsl-plugin it was not exposed so I added it.
I added a new function "network" in docker context and exposed it in the wrapper.

Picture of the Maven clean install (after I added some tests for the new ability):

Picture of the actual use of the plugin after installing it on testing Jenkins:

https://issues.jenkins.io/browse/JENKINS-70542

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[guetta14]

Hi @basil, This is a really minor enhancement that would mean a lot to us, can you please review it and give your opinion?

[basil]

Thanks for the PR!