diff --git a/changelog.d/6390.doc b/changelog.d/6390.doc
new file mode 100644
index 0000000000..093411bec1
--- /dev/null
+++ b/changelog.d/6390.doc
@@ -0,0 +1 @@
+Add build instructions to the docker readme.
\ No newline at end of file
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.