Docker image size grew a lot in Synapse v0.33.5.1

[spantaleev]

#3834 was supposed to reduce the size of the Docker images, and it seems like it did, initially.

The size of v0.33.4 (before #3834) was 178 MB.
After #3834, v0.33.5rc1 got released and its size got reduced to 115 MB.

.. but then v0.33.5.1 got released and its size exploded to 289 MB for some reason.

For v0.33.5, it seems like there's quite a lot of junk in the Docker image's /synapse directory:

# du -hs /synapse/* | grep M
....
158.4M	/synapse/env
156.8M	/synapse/env3
122.4M	/synapse/homeserver.db.bak0170829
139.2M	/synapse/homeserver.db.bak20171001
24.1M	/synapse/homeserver.db.old
143.1M	/synapse/homeserver.db.snapshot
106.8M	/synapse/homeserver0.db
8.3M	/synapse/logs
1.3M	/synapse/media_store
20.3M	/synapse/py3env
1.7M	/synapse/server2
1.6M	/synapse/tests

That's a lot of stuff that doesn't seem like it should be there..

[richvdh]

Sadly nobody had documented the process for making docker images, and I guessed and got it wrong. Now fixed.