2 files changed, 15 insertions, 3 deletions
diff --git a/docker/README.md b/docker/README.md
index 24dfa77dcc..9f112a01d0 100644
--- a/docker/README.md
+++ b/docker/README.md
@@ -130,3 +130,15 @@ docker run -it --rm \
 This will generate the same configuration file as the legacy mode used, but
 will store it in `/data/homeserver.yaml` instead of a temporary location. You
 can then use it as shown above at [Running synapse](#running-synapse).
+
+## Building the image
+
+If you need to build the image from a Synapse checkout, use the following `docker
+ build` command from the repo's root:
+ 
+```
+docker build -t matrixdotorg/synapse -f docker/Dockerfile .
+```
+
+You can choose to build a different docker image by changing the value of the `-f` flag to
+point to another Dockerfile.
diff --git a/docker/start.py b/docker/start.py
index 6e1cb807a1..97fd247f8f 100755
--- a/docker/start.py
+++ b/docker/start.py
@@ -169,11 +169,11 @@ def run_generate_config(environ, ownership):
     # log("running %s" % (args, ))
 
     if ownership is not None:
-        args = ["su-exec", ownership] + args
-        os.execv("/sbin/su-exec", args)
-
         # make sure that synapse has perms to write to the data dir.
         subprocess.check_output(["chown", ownership, data_dir])
+
+        args = ["su-exec", ownership] + args
+        os.execv("/sbin/su-exec", args)
     else:
         os.execv("/usr/local/bin/python", args)