Fix and modernise the controller Dockerfile

[excieve]

Currently the controller Dockerfile is referencing some goproxy directory on the image that doesn't even exist:

tyk-k8s/docker/controller/Dockerfile

Line 8 in 21d094c

CMD ["/opt/goproxy/tyk-k8s", "start"]

The binary is installed in the /opt/tyk-k8s directory instead.

Moreover, the Dockerfile itself could use some love:

• Move to multi-stage builds
• Use a smaller base image (e.g. Debian slim or Alpine).
• Use an entrypoint
• Run the process as a non-root user