ref: Stop building local images for Sentry services (#834)

We used to build local images for Sentry services to be able to
include required plugins in the image. With this change we instead
do this in a custom entrypoint script and use the volume /data
to store the plugins permanently.

This should resolve many issues people have around building local
images and pushing them to places like private repositories or swarm
clusters.

This is not 100% compatible with the old way but it should still be
a mostly transparent change to many folks.