Proposal for changes in official Docker image

Hi!

First of all, thanks for the great app :)

I started using PgAdmin with docker image (dpage/pgadmin4) a few weeks
ago, however I thought that it had some issues, so I decided to make
my own image. Some of the advantages:

- Use alpine linux instead of centos to greatly reduce image size
(170MB vs 560MB)
- Use lightweight pure-python HTTP server waitress instead of heavy
apache/mod_wsgi
- Use python 3.6

You can test the image at https://hub.docker.com/r/maksbotan/pgadmin4/
Readme contains more detailed explanation and usage instructions.

The Dockerfile is hosted at github: https://github.com/maksbotan/pgadmin4_docker

If you find my work useful, I'd love to make a contribution with these
scripts, after some discussion with pgadmin developers and further
improvements.

Looking forward for an answer,
Maxim.