Sidecar docker containers for declarative

The "sidecar container" pattern is a popular one for setting up dependent containers for running tests/builds. 

It would be nice to have support for this pattern in declarative to avoid the need to drop to script. 

To investigate...

In scope: 

• Be able to specify sidecar containers, their network/port sharing with main build, in agent section
• Cleanup of containers
• Formalise the patterns shown: https://jenkins.io/doc/book/pipeline/docker/#running-sidecar-containers in declarative, essentially

testing: 

• Should be able to be fully automated using ci.jenkins.io 
• check sidecar containers are shut down in all scenarios possible