docker: Add hooks for arm build

https://stackoverflow.com/questions/54578066/how-to-build-a-docker-image-on-a-specific-architecture-with-docker-hub
docker/hub-feedback#1261
https://aquarat.co.za/tag/dockerhub/

docker/hooks/build

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+docker build \
+    --file "${DOCKERFILE_PATH}" \
+    --build-arg BUILD_DATE="$(date -u +"%Y-%m-%dT%H:%M:%SZ")" \
+    --build-arg VCS_REF="$(git rev-parse --short HEAD)" \
+    --tag "$IMAGE_NAME" \
+    .

docker/hooks/post_checkout

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+BUILD_ARCH=$(echo "${DOCKERFILE_PATH}" | cut -d '.' -f 2)
+
+[ "${BUILD_ARCH}" == "Dockerfile" ] && \
+    { echo 'qemu-user-static: Download not required for current arch'; exit 0; }
+
+QEMU_USER_STATIC_ARCH=$([ "${BUILD_ARCH}" == "armhf" ] && echo "${BUILD_ARCH::-2}" || echo "${BUILD_ARCH}")
+QEMU_USER_STATIC_DOWNLOAD_URL="https://github.com/multiarch/qemu-user-static/releases/download"
+QEMU_USER_STATIC_LATEST_TAG=$(curl -s https://api.github.com/repos/multiarch/qemu-user-static/tags \
+    | grep 'name.*v[0-9]' \
+    | head -n 1 \
+    | cut -d '"' -f 4)
+
+curl -SL "${QEMU_USER_STATIC_DOWNLOAD_URL}/${QEMU_USER_STATIC_LATEST_TAG}/x86_64_qemu-${QEMU_USER_STATIC_ARCH}-static.tar.gz" \
+    | tar xzv

docker/hooks/pre_build

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+BUILD_ARCH=$(echo "${DOCKERFILE_PATH}" | cut -d '.' -f 2)
+
+[ "${BUILD_ARCH}" == "Dockerfile" ] && \
+    { echo 'qemu-user-static: Registration not required for current arch'; exit 0; }
+
+docker run --rm --privileged multiarch/qemu-user-static:register --reset