Added multiplatform build and push instructions

README.md

@@ -9,4 +9,38 @@ To build the container to an image called "cmatrix-demo", run
 
 ```sh
 docker build -t cmatrix-demo .
-```
+```
+
+## Multi-platform builds
+
+To do a build for multiple architectures, run:
+
+```sh
+sudo docker buildx create --name buildx-multi-arch
+sudo docker buildx use buildx-multi-arch
+```
+
+Then, to build it (e.g. for amd64 and arm64/v8):
+
+```sh
+sudo docker buildx build --no-cache --platform linux/amd64,linux/arm64/v8 -t cmatrix-demo .
+```
+
+**Please note that this will not load the images into the local image store (because it's obviously hard to run images built for the wrong architecture)! For uploading this to a container registry, see below.**
+
+
+## Pushing the container to a registry
+
+As you've got everything working, maybe it's time to share the work! This can be done by uploading the built image to a docker container registry, e.g. [Docker Hub](https://hub.docker.com/).
+
+First, sign into your Docker Hub account with your local docker installation (to create an account head to [https://app.docker.com/signup](https://app.docker.com/signup) in your favourite browser). The procedure is self-explaining:
+
+```sh
+docker login
+```
+
+To build and push the image directly to Docker Hub, use the following command (differs from the one above only in the --push option):
+
+```sh
+sudo docker buildx build --no-cache --platform linux/amd64,linux/arm64/v8 -t <USER>/cmatrix-demo . --push
+```