Document the --dev flag for src/ci/docker/run.sh

Author: clubby789

src/ci/docker/README.md

@@ -26,6 +26,14 @@ DEPLOY=1 ./src/ci/docker/run.sh x86_64-gnu
 while locally, to the `obj/$image_name` directory. This is primarily to prevent
 strange linker errors when using multiple Docker images.
 
+## Local Development
+
+To get an interactive shell in the Docker image, pass the `--dev` flag, e.g.:
+
+```
+./src/ci/docker/run.sh --dev x86_64-gnu
+```
+
 ## Filesystem layout
 
 - Each host architecture has its own `host-{arch}` directory, and those