Docker images shouldn't run as root

[wochinge]

Same as our rasa and rasa-x images, the sdk image should not run as root user.
To fix this, we can build upon the work from #115 . Among the things which have to be wrapped up

• use group id 0
• make sure all used folders are accessible for this group 0 (we have to make it accessible to the group and not to the user, since OpenShift runs with a random user but with a fixed group id 0)

[alwx]

Also we need to check this issue after we actually manage to run all the images as non-root:
RasaHQ/rasa-x-helm#18

[alwx]

Linked issues: #145 & RasaHQ/rasa#5223

(new Dockerfiles there use user 1001)