Add dockerExecCommand setting

[rbellamy]

Per discussion in #903

When building docker images on CentOS, Fedora or RHEL, the permissions on /var/run/docker.socket have restricted access, preventing Docker build and push commands as a normal user from executing without severely compromising the system.

See http://www.projectatomic.io/blog/2015/08/why-we-dont-let-non-root-users-run-docker-in-centos-fedora-or-rhel/

Therefore, I've added a new setting which supports replacing the default docker shell exec command so that I can instead use sudo docker (which is the recommended method for executing Docker on CentOS, Fedora and RHEL), or completely replace the docker command with a path to a shell script.

With this new setting and the regular dockerBuildCommand and dockerBuildOptions, you get the following basics:

dockerBuildCommand := dockerExecCommand.value ++ Seq("build") ++ dockerBuildOptions.value ++ Seq(".")
// default
dockerExecCommand := Seq("docker")
// custom
dockerExecCommand := Seq("sudo", "docker")

What I don't want to do is run SBT as root - this will cause the working directory, and the ivy cache to become polluted with artifacts that are owned by root. It's best to limit the use of escalated privileges to just those commands that require them to avoid this pollution.

So, to avoid:

sudo sbt docker:publishLocal
sbt clean <=== will fail because "target" is now owned by root

[muuki88]

Thanks. Looks perfect.