Add support for multiarch container images (amd64/arm64)

Container images built with support for the ARM architecture
perform much better on Apple Silicon laptops.

docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM apluslms/service-base:django-1.17
+FROM --platform=$TARGETPLATFORM apluslms/service-base:django-1.18
 
 # Set container related configuration via environment variables
 ENV CONTAINER_TYPE="radar" \

docker/hooks/build

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+docker run --privileged --rm tonistiigi/binfmt --install all
+
+docker buildx create --use default
+
+docker buildx build \
+--push \
+--tag "$DOCKER_REPO":"$DOCKER_TAG" \
+--platform linux/amd64,linux/arm64 \
+--file Dockerfile ..

docker/hooks/push

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+# This hook is empty because the image was pushed to the registry in the build hook