fix issues in .golangci.yml and add validation check

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

hack/dockerfiles/lint.Dockerfile

@@ -13,18 +13,24 @@ FROM --platform=$BUILDPLATFORM tonistiigi/xx:${XX_VERSION} AS xx
 FROM --platform=$BUILDPLATFORM golang:${GO_VERSION}-alpine AS golang-base
 RUN apk add --no-cache git gcc musl-dev
 
-FROM golang-base AS lint
+FROM golang-base AS lint-base
 ENV GOFLAGS="-buildvcs=false"
 ARG GOLANGCI_LINT_VERSION
 RUN wget -O- -nv https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s v${GOLANGCI_LINT_VERSION}
 COPY --link --from=xx / /
 WORKDIR /go/src/github.com/docker/buildx
 ARG TARGETPLATFORM
+
+FROM lint-base AS lint
 RUN --mount=target=/go/src/github.com/docker/buildx \
     --mount=target=/root/.cache,type=cache,id=lint-cache-$TARGETPLATFORM \
     xx-go --wrap && \
     golangci-lint run
 
+FROM lint-base AS validate-golangci
+RUN --mount=target=/go/src/github.com/docker/buildx \
+  golangci-lint config verify
+
 FROM golang-base AS gopls
 RUN apk add --no-cache git
 ARG GOPLS_VERSION