Add Makefile and bin/ scripts for one-command multi-arch release

make release builds, smoke-tests and pushes amd64 (on fsn1app1 via
DOCKER_HOST) and arm64 (local), joins them into latest and dated
multi-arch manifests, and commits the day's logs. Logic lives in bin/;
the Makefile only wires dependencies so -j2 runs the arches in parallel.

bin/lib.sh

@@ -0,0 +1,32 @@
+# Shared setup for bin/ scripts. Source after `set -euo pipefail`.
+#
+# Expects IMAGE and DATE in the environment (the Makefile exports them) and
+# takes the target arch as $1 where relevant.
+
+: "${IMAGE:=git.eeqj.de/sneak/baseimage}"
+: "${DATE:=$(date +%Y-%m-%d)}"
+
+LOGDIR="logs/$DATE"
+mkdir -p "$LOGDIR"
+
+# Point docker at the daemon for the given arch. DOCKER_HOST_<arch> empty or
+# unset means the local daemon.
+use_arch_host() {
+    local arch="$1" var host
+    var="DOCKER_HOST_${arch}"
+    host="${!var:-}"
+    if [ -n "$host" ]; then
+        export DOCKER_HOST="$host"
+    else
+        unset DOCKER_HOST
+    fi
+    local actual
+    actual="$(docker version --format '{{.Server.Arch}}')"
+    if [ "$actual" != "$arch" ]; then
+        echo "error: daemon for $arch (${host:-local}) reports arch $actual" >&2
+        return 1
+    fi
+}
+
+dev_tag() { echo "baseimage:dev-$1"; }
+arch_tag() { echo "$IMAGE:$1"; }