Updates to Docker image

[rcoup]

@rouault I'm not 100% clear on what the Docker image was intended for, since it didn't expose/run any services; maybe just a clean build process to obtain task.zip?

Anyhow, I've made it additionally actually run a COG validator service

$ docker build -t cog_validator .
$ docker run --rm -p 5000:5000 cog_validator

Now runs the service on http://localhost:5000/

Additionally:

• Enable customisable listen interface via environment variable LISTEN
• Enable customisable upload size limit via environment variable COG_LIMIT
• Update to GDAL 2.3.1
• Enable ZSTD compression support
• Redirect request to / to /html rather than 404ing