Update Dockerfile

Dockerfile

@@ -9,8 +9,11 @@ ENV CGO_ENABLED 0
 ARG ACTIONLINT_VER=
 RUN go build -v -ldflags "-s -w -X github.com/rhysd/actionlint.version=${ACTIONLINT_VER}" ./cmd/actionlint
 
+FROM koalaman/shellcheck-alpine:stable as shellcheck
+
 FROM alpine:${ALPINE_VER}
 COPY --from=builder /go/src/app/actionlint /usr/local/bin/
-RUN apk add --no-cache shellcheck py3-pyflakes
+COPY --from=shellcheck /bin/shellcheck /usr/local/bin/shellcheck
+RUN apk add --no-cache py3-pyflakes
 USER guest
 ENTRYPOINT ["/usr/local/bin/actionlint"]