hack: add docs generation/validation

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

hack/dockerfiles/docs.Dockerfile

@@ -0,0 +1,29 @@
+# syntax = docker/dockerfile:1.2
+
+FROM golang:1.16-alpine AS docsgen
+WORKDIR /src
+RUN --mount=target=. \
+  --mount=target=/root/.cache,type=cache \
+  go build -mod=vendor -o /out/docsgen ./docs/docsgen
+
+FROM alpine AS gen
+RUN apk add --no-cache rsync git
+WORKDIR /src
+COPY --from=docsgen /out/docsgen /usr/bin
+RUN --mount=target=/context \
+  --mount=target=.,type=tmpfs,readwrite  \
+  rsync -a /context/. . && \
+  docsgen && \
+  mkdir /out && cp -r docs/reference /out
+
+FROM scratch AS update
+COPY --from=gen /out /out
+
+FROM gen AS validate
+RUN --mount=target=/context \
+  --mount=target=.,type=tmpfs,readwrite  \
+  rsync -a /context/. . && \
+  git add -A && \
+  rm -rf docs/reference/* && \
+  cp -rf /out/* ./docs/ && \
+  ./hack/validate-docs check