Update documentation about the name of the container image

[tfr42]

Currently the documentation refers to docker pull deegree/deegree-ogcapi-postgres:latest as the name of the image.
It shall be docker pull deegree/deegree-ogcapi:latest. This image contains all supported SQL dialects provided by the deegree core frameword: postgresql, oracle, mssql.
see
https://hub.docker.com/repositories/deegree