portal-api does not react on SIGTERM, should shut down

[DonMartin76]

To make killing containers e.g. in Kubernetes, the portal-api should react to SIGTERM (and SIGINT) and shut down itself when it receives this signal.

This is especially needed when running in Kubernetes, as the scheduler first sends a SIGTERM, then, after 30s, a SIGKILL. If a container is to shut down, it should do so immediately (but gracefully) when it receives SIGTERM.

Side note: The other containers (mailer, chatbot, kong adapter) do.

[DonMartin76]

Reason: portal-api uses a startup script which gets PID 1. Measure: Use https://github.com/Yelp/dumb-init