Add working build command for docker image

changelog.d/6390.doc

@@ -0,0 +1 @@
+Add build instructions to the docker readme.

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.