validate and update vendor target

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

dockerfiles/vendor.Dockerfile

@@ -0,0 +1,37 @@
+# syntax=docker/dockerfile:1
+
+ARG GO_VERSION=1.17
+
+FROM golang:${GO_VERSION}-alpine AS base
+RUN apk add --no-cache git rsync
+WORKDIR /src
+
+FROM base AS vendored
+RUN --mount=target=/context \
+  --mount=target=.,type=tmpfs  \
+  --mount=target=/go/pkg/mod,type=cache <<EOT
+set -e
+rsync -a /context/. .
+go mod tidy
+go mod vendor
+mkdir /out
+cp -r go.mod go.sum vendor /out
+EOT
+
+FROM scratch AS update
+COPY --from=vendored /out /out
+
+FROM vendored AS validate
+RUN --mount=target=/context \
+  --mount=target=.,type=tmpfs <<EOT
+set -e
+rsync -a /context/. .
+git add -A
+rm -rf vendor
+cp -rf /out/* .
+if [ -n "$(git status --porcelain -- go.mod go.sum vendor)" ]; then
+  echo >&2 'ERROR: Vendor result differs. Please vendor your package with "make vendor"'
+  git status --porcelain -- go.mod go.sum vendor
+  exit 1
+fi
+EOT