Write Dockerfile for data plane

[kate-osborn]

Parent issue: #292

Write a Dockerfile for the data plane. Consider the following questions when writing:

Will the agent be PID 1 that will also start NGINX?
What will happen if NGINX master fails (terminates) ? What will happen if the agent fails?
How to gracefully shutdown NGINX?

A/C:

• The container runs both nginx and agent processes
• The container is as small as possible
• Add build step to Makefile

Aha! Link: https://nginx.aha.io/features/NKG-34

[pleshakov]

do we need to do anything with CI (like building/publishing new images) ?

[brianehlert]

do we need to do anything with CI (like building/publishing new images) ?

We should, but we can tie that to some release preparation later on.
For now, consider the NIC pattern here.