Images are no longer built locally, Instead, they are downloaded from
docker hub. This completely changes config file organisation. In
particular, we no longer copy configuration files to the original docker
image.
XModule could not be found because the compiled package could not be
found in edx-platform/common/lib/xmodule. This was due to the fact that
build operations realised after a VOLUME statement are ignored if they
affect the content of this volume (although it's not mounted during
build, of course). This is not a bug, it's a feature of the VOLUME
statement :)
