Do not use PORT 80 by default

[nyurik]

Using port 80 by default requires the docker to run as ROOT. Please change it to some other port above 1024, e.g. 8080.

Example of failure: docker run -it --rm -u $(id -u):$(id -g) -p 8080:80 maptiler/tileserver-gl -- tileserve-gl will not be able to open the port.

A less-than-ideal workaround would be to add --port param, e.g.

docker run -it --rm -u $(id -u):$(id -g) -p 8080:8080 maptiler/tileserver-gl --port 8080

Also note that EXPOSE 80 in Dockerfile would need to be updated (technically it is not required as it is mostly for documentation purposes from my understanding)

[StefanBrand]

Related: #517