Runs as root by default

[nigel-gott]

Hi, currently I believe mjml-server's image runs as root by default which is not desirable. The base image (node) used has a page with best practices showing how instead it could run as the node user which is more secure:
https://github.com/nodejs/docker-node/blob/main/docs/BestPractices.md#non-root-user
What do you think about making this change in mjml-server also?

[adrianrudnik]

Hi there, yes I want to move it to a non-root user soon, through it will introduce a BC break because the default listening port will change from 80 to 8080 as I can no longer use the privileged port.

[mmulqueen]

If anyone comes across this, here are the basic modifications needed to run as non-root DigitalGrads@1c33bc7 (listens on port 8080)

I won't submit a pull request because as @adrianrudnik states, it's a breaking change.

P.S. Thanks for making this Docker image.