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

templates/project/_shared/docker/test/Dockerfile

@@ -0,0 +1,11 @@
+FROM debian:bookworm-slim
+
+ENV DEBIAN_FRONTEND=noninteractive
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+       ca-certificates git bash curl jq yq \
+       bats \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /work
+