The tags are named after the PostgreSQL versions; PostgreSQL 9.4 is using Postgis 2.1, 9.5 is using 2.2.
Here is a sample Dockerfile for using it:
FROM camptocamp/postgis:9.5
MAINTAINER Camptocamp "info@camptocamp.com"
ENV POSTGRES_USER www-data
ENV POSTGRES_DB toto
ADD *.sql /docker-entrypoint-initdb.d/
Name your SQL files starting with a 2 digit number prefix starting from 01_
to ensure your scripts are started after the ones from the
camptocamp/postgis
image.
See the postgres image documentation for more details: https://hub.docker.com/_/postgres/