Run the server as an unprivileged user

Dockerfile

 FROM python:2-alpine
 
-RUN apk add --no-cache --virtual .nacl_deps build-base libffi-dev zlib-dev openssl-dev libjpeg-turbo-dev linux-headers postgresql-dev
+RUN apk add --no-cache --virtual .nacl_deps su-exec build-base libffi-dev zlib-dev openssl-dev libjpeg-turbo-dev linux-headers postgresql-dev
 
 COPY . /synapse
 

contrib/docker/start.py

@@ -9,14 +9,16 @@ convert = lambda src, dst, environ: open(dst, "w").write(jinja2.Template(open(sr
 mode = sys.argv[1] if len(sys.argv) > 1 else None
 environ = os.environ.copy()
 
+# Check mandatory parameters and build the base start arguments
 if "SYNAPSE_SERVER_NAME" not in environ:
     print("Environment variable SYNAPSE_SERVER_NAME is mandatory, exiting.")
     sys.exit(2)
 
+permissions = "{}:{}".format(environ.get("UID", 991), environ.get("GID", 991))
 args = ["python", "-m", "synapse.app.homeserver",
-        "--server-name", os.environ.get("SYNAPSE_SERVER_NAME"),
-        "--report-stats", os.environ.get("SYNAPSE_REPORT_STATS", "no"),
-        "--config-path", os.environ.get("SYNAPSE_CONFIG_PATH", "/compiled/homeserver.yaml")]
+        "--server-name", environ.get("SYNAPSE_SERVER_NAME"),
+        "--report-stats", environ.get("SYNAPSE_REPORT_STATS", "no"),
+        "--config-path", environ.get("SYNAPSE_CONFIG_PATH", "/compiled/homeserver.yaml")]
 
 # Generate any missing shared secret
 for secret in ("SYNAPSE_REGISTRATION_SHARED_SECRET", "SYNAPSE_MACAROON_SECRET_KEY"):
@@ -35,4 +37,5 @@ if mode == "generate":
 # In normal mode, generate missing keys if any, then run synapse
 else:
     subprocess.check_output(args + ["--generate-keys"])
-    os.execv("/usr/local/bin/python", args)
+    subprocess.check_output(["chown", "-R", permissions, "/data"])
+    os.execv("/sbin/su-exec", ["su-exec", permissions] + args)