Update docker build instructions

Author: chaoflow

README.md

@@ -184,7 +184,11 @@ There are three ways of building the required restream binary for streaming the
   You can use the [toltec toolchain docker images](https://github.com/toltec-dev/toolchain) to build a restream binary compatible with the reMarkable.
 
     ```
-    docker run --rm -v $(pwd):/project -v /project/.cargo -w "/project" ghcr.io/toltec-dev/rust:latest cargo build --release --target=armv7-unknown-linux-gnueabihf
+    docker run --rm -v "$(pwd)":/project -v /project/.cargo -w /project \
+      ghcr.io/toltec-dev/rust:latest bash -c '
+        source /opt/x-tools/switch-arm.sh
+        cargo build --release --target=armv7-unknown-linux-gnueabihf
+      '
     ```
 
 - **Using the reMarkable toolchain:**