add docker build image command for building multi architecture image

docs/build-image.md

@@ -0,0 +1,15 @@
+To build a multi-architecture image, you need to use `buildx`.
+
+Here's the command to build and push the images from a Mac M1:
+
+1) First create a new buildx environment (or context). This is only necessary for the first time.
+
+```bash
+docker buildx create --use
+```
+
+2) Build and push the image. Replace `simplelogin/name:tag` by the correct docker image name and tag.
+
+```bash
+docker buildx build --platform linux/amd64,linux/arm64 --push -t simplelogin/name:tag
+```