The problem is that the default location for the data directory is configured as a VOLUME in the base image. It seems the maintainer changed this shortly after you posted your question. Changes to the file system within VOLUME directories are ignored by Docker, as far as image commits are concerned.
I ran into this problem with the stackbrew Postgres image. To be able to create a new image containing postgres + custom schema, a new non-VOLUME data directory is needed. Postgres also needs to be configured to use this directory. In stackbrew's version, this is as simple as overriding the PGDATA environment variable in the custom Dockerfile:
FROM postgres
# Original 'PGDATA' is /var/lib/postgresql/data
RUN mkdir -p /var/lib/pgdata
ENV PGDATA /var/lib/pgdata
# RUN ... logic to create schema