Create a Freestyle job which builds in a docker container from CloudBees Docker Custom Build Environment Plugin.

Add Credential Binding to the job: "SSH User Private Key".

This has multiple fields you can set: Key File Variable, Passphrase Variable (optional), Username Variable (optional)

If you don't set those optional variables in the job itself, then the build crashes.   The output is shown below. The crash does not happen if you have a regular job without Docker. The crash does not happen if you have Docker but you specify all the so-called "optional" variables.

The problem is that if someone is not familiar with this bug, it may not be obvious why the build is failing.

 Console Output
Started by user Sam
Running as SYSTEM
[EnvInject] - Loading node environment variables.
Building on master in workspace /var/lib/jenkins/workspace/test1
invalid argument "=***" for "-e, --env" flag: invalid environment variable: =***
See 'docker run --help'.
FATAL: Failed to run docker image
java.lang.RuntimeException: Failed to run docker image
at com.cloudbees.****.plugins.docker_build_env.Docker.runDetached(Docker.java:239)
at com.cloudbees.****.plugins.docker_build_env.DockerBuildWrapper.startBuildContainer(DockerBuildWrapper.java:212)
at com.cloudbees.****.plugins.docker_build_env.DockerBuildWrapper.setUp(DockerBuildWrapper.java:185)
at hudson.model.Build$BuildExecution.doRun(Build.java:157)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:504)
at hudson.model.Run.execute(Run.java:1894)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:428)
Finished: FAILURE