Allow to supply license id and key as files

[hatzlj]

Instead of supplying the license id and key directly via environment variables (LICENSE_ID, LICENSE_KEY) it would be helpful to alternatively allow supplying them also as files to the container and map the contents of these files to the LICENSE_ID and LICENSE_KEY environment variables.

A solution could allow two additional environment variables like LICENSE_ID_FILE and LICENSE_KEY_FILE. The startup script maps the contents of the files specified by these variables to the LICENSE_ID and LICENSE_KEY variables.

Usage with docker secrets could then look like:

services:
  app:
    image: my-mendix-app:latest
    secrets:
      - license-id
      - license-key
    environment:
      - LICENSE_ID_FILE=/run/secrets/license-id
      - LICENSE_KEY_FILE=/run/secrets/license-key
secrets:
  license-id:
    file: ./license-id.txt
  license-key:
    file: ./license-key.txt

As stated by the docker compose documentation supplying sensitive information via environment variables my lead to unintentional information exposure, whereas secrets can only be accessed by services that have granted access explicitly via the secrets attribute in the config.

[hatzlj]

I've prepared a PR that implements this additional functionality (see #223)