Add instructions for building docker image locally to README.md

Fixes #8442
Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>

.gitignore

@@ -23,3 +23,4 @@ npm_licenses.tar.bz2
 /web/ui/assets_vfsdata.go
 
 /vendor
+/.build

README.md

@@ -91,9 +91,19 @@ The Makefile provides several targets:
   * *test-short*: run the short tests
   * *format*: format the source code
   * *vet*: check the source code for common errors
-  * *docker*: build a docker container for the current `HEAD`
   * *assets*: build the new experimental React UI
 
+### Building the Docker image
+
+The `make docker` target is designed for use in our CI system.
+You can build a docker image locally with the following commands:
+
+    $ make promu
+    $ promu crossbuild -p linux/amd64
+    $ make common-docker-amd64
+
+*NB* if you are on a Mac, you will need [gnu-tar](https://formulae.brew.sh/formula/gnu-tar).
+
 ## React UI Development
 
 For more information on building, running, and developing on the new React-based UI, see the React app's [README.md](https://github.com/prometheus/prometheus/blob/master/web/ui/react-app/README.md).