Allow configurable sagittarius host in nextjs proxy

[Taucher2003]

When using sculptor in the docker container, the graphql proxy doesn't work because it tries to proxy to a sagittarius running inside the same container. Instead of hardcoding localhost in the proxy, the proxy url should be overridable with an environment variable.

[Taucher2003]

This is no longer needed. For development the sagittarius host is configurable with an environment variable and in docker the routing is handled by a nginx container