build: Fix syntax case

Signed-off-by: Chris Crone <christopher.crone@docker.com>

Dockerfile

@@ -95,7 +95,7 @@ COPY --from=make-cli /out/* .
 FROM scratch AS cross
 COPY --from=make-cross /out/* .
 
-FROM base as test
+FROM base AS test
 ENV CGO_ENABLED=0
 ARG BUILD_TAGS
 ARG GIT_TAG
@@ -106,11 +106,11 @@ RUN --mount=target=. \
     GIT_TAG=${GIT_TAG} \
     make -f builder.Makefile test
 
-FROM base as check-license-headers
+FROM base AS check-license-headers
 RUN go get -u github.com/kunalkushwaha/ltag
 RUN --mount=target=. \
     make -f builder.Makefile check-license-headers
 
-FROM base as check-go-mod
+FROM base AS check-go-mod
 COPY . .
 RUN make -f builder.Makefile check-go-mod