Use Docker Buildx for multi-arch builds

The bitwarden_rs code is still cross-compiled exactly as before, but Docker
Buildx is used to rewrite the resulting Docker images with correct platform
metadata (reflecting the target platform instead of the build platform).
Buildx also now handles building and pushing the multi-arch manifest lists.

hooks/pre_build

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+set -ex
+
+# Print some environment info in case it's useful for troubleshooting.
+id
+pwd
+df -h
+env
+docker info
+docker version
+
+# Install build dependencies.
+deps=(
+    jq
+)
+apt-get update
+apt-get install -y "${deps[@]}"