scaffold: add project CI and Docker templates under templates/project/_shared; update proposal and plan to include them

templates/project/_shared/scripts/test.docker.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+here="$(cd "$(dirname "$0")" && pwd)"
+root="$(cd "$here/.." && pwd)"
+
+compose_file="$root/docker/compose.yml"
+
+echo "[docker] Running tests via docker compose"
+docker compose -f "$compose_file" up --build --abort-on-container-exit --remove-orphans
+docker compose -f "$compose_file" down -v --remove-orphans
+