Allow to add arguments on nginx-proxy docker run

[clesquir]

e.g.:

proxy: {
   shared: {
        dockerArgs: [
            '-p 8080:8080'
        ]
    }
}

[zodern]

Thanks for creating this @clesquir.

Two things:

1. Since changing the docker arguments effects every app using the reverse proxy, it should be part of reconfigShared.
2. Move the option to proxy.shared.dockerArgs.

Once #706 is implemented there will be an easier way to add ports, but this PR has many additional uses such as adding networks, volumes, using a custom nginx template, etc.

[clesquir]

Please let me know if there is anything else to be modified!