ci: Add multi-arch Docker build/publish and tagged release workflow

Build and push linux/amd64 + linux/arm64 images to GHCR on every push
to master and on v* tags. On v* tags, also generate a changelog from
conventional commits and create a GitHub release.

.github/generate_changelog.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Usage: ./generate_changelog.sh [<git-log-range>]
+#
+# Groups commits by their conventional-commit prefix (feat, fix, ...) into
+# sections. Commits within each section are sorted oldest-first. Commits
+# that don't match a known prefix land in an "Other" section.
+
+range="${1:-}"
+
+types="feat fix refactor perf docs test ci build chore"
+
+label_for() {
+  case "$1" in
+    feat) echo "Features" ;;
+    fix) echo "Fixes" ;;
+    refactor) echo "Refactoring" ;;
+    perf) echo "Performance" ;;
+    docs) echo "Documentation" ;;
+    test) echo "Tests" ;;
+    ci) echo "CI" ;;
+    build) echo "Build" ;;
+    chore) echo "Chores" ;;
+    *) echo "Other" ;;
+  esac
+}
+
+commits=$(git log ${range} --reverse --pretty='format:%s')
+known_pattern="^(feat|fix|refactor|perf|docs|test|ci|build|chore): "
+
+first_section=1
+for type in $types other; do
+  if [ "$type" = "other" ]; then
+    entries=$(printf '%s\n' "$commits" | grep -v -E "$known_pattern" | sed 's/^/- /' || true)
+  else
+    entries=$(printf '%s\n' "$commits" | grep -E "^${type}: " | sed -E "s/^${type}: /- /" || true)
+  fi
+
+  [ -z "$entries" ] && continue
+  [ "$first_section" -eq 0 ] && echo
+  first_section=0
+
+  printf '### %s\n\n%s\n' "$(label_for "$type")" "$entries"
+done