Reduce the amount of stuff we send in the docker context (#5564)

this makes docker builds a bit faster.

.dockerignore

@@ -1,9 +1,13 @@
-Dockerfile
-.travis.yml
-.gitignore
-demo/etc
-tox.ini
-.git/*
-.tox/*
-debian/matrix-synapse/
-debian/matrix-synapse-*/
+# ignore everything by default
+*
+
+# things to include
+!docker
+!scripts
+!synapse
+!MANIFEST.in
+!README.rst
+!setup.py
+!synctl
+
+**/__pycache__

changelog.d/5564.misc

@@ -0,0 +1 @@
+Reduce the amount of stuff we send in the docker context.