Added a zoonavigator-api user to Dockerfile

[rajha-korithrien]

This pull request addresses issues raised in elkozmon/zoonavigator#36. What it does:

• Causes zoonavigator-api to run as a non-root user inside a Docker container

Why is it needed:

• Some security environments only allow Docker images that run as non-root and there is no need for zoo-navigator to run as root.

Potential risks:

• None - this should be a completely transparent change to end users

[elkozmon]

Looks good, thank you!