Add an ability to specify entrypoint for dockerimage tool in Devfile

Description

There are cases when a user wants to use some official docker image in dockerimage tool but default entrypoint is not suitable for his needs. In this case, it is needed to have an ability to specify entrypoint and arguments for dockerimage tool in Devfile. The proposal is to add two optional fields command and arguments which type is string array. It’s the same as Kubernetes Container model has https://docs.openshift.com/container-platform/3.5/rest_api/kubernetes_v1.html#v1-container. It would look like the following

  - name: nodejs
    type: dockerimage
    image: node:0.10.40
    command: ['/bin/sh', '-c']
    args: ['tail -f /dev/null']

[l0rd]

👍 for containerCommand and containerArgs

[garagatyi]

👍 to use attributes until we remove workspace config and start using devfile directly. I’m OK with the proposed names