Docker images security considerations

Currently used MJML Server Docker image (https://hub.docker.com/r/liminspace/mjml-tcpserver) is provided by a 3rd party, it is not official, and it has not been updated for a long time, which introduces security issues.

Forking and automated builds (e.g. using GitLab CI/CD) should be considered instead.

Also, one should consider changing Postgres'/Redis' images to use tags like 6-buster and 11 respectively (or rather 6-alpine and 11-alpine respectively to reduce resources usage) not to use EOL/vulnerable images or software itself, but keeping a major version the same.