runCmds are only executable with /bin/sh set

[MeiSign]

Description

I am trying to create a docker image with RUN parameters and a linux distribution as FROM param, that hasn't linked /bin/sh. Dockerfiles support two RUN syntaxes to set the executable that should your command or to use the standard shell (/bin/sh). (https://docs.docker.com/engine/reference/builder/#run).

When you are in the situation that /bin/sh is not linked you are in a deadlock situation with the plugin. You need a RUN cmd to link the shell but you can't execute the link command.

I can think of two possible solutions:

1. Extend the runCmd section to support the exec syntax
2. Support the SHELL param to set a standard shell without linking it per RUN parameter (https://docs.docker.com/engine/reference/builder/#shell)

If I didn't miss something, I can make a pull request if you tell me which of the options you would prefer.

[MeiSign]

I have added a pull request with the implementation of the second option.
The runCommands have already the optimise option and I don't wanted to complicate this setting further.

[rhuss]

I agree that option 2 is probably simpler (remember, you have always the option to use a plain Dockerfile if advanced Dockerfile options are required).

The PR looks good, with some minor change request (see over there).