Add RELEASE_VERSION build-arg to the image 📟

This will allow to build release version of the image, by default the
arg is empty, making it a `dev` version.

Signed-off-by: Vincent Demeester <vdemeest@redhat.com>

contrib/tkn-image/Dockerfile

@@ -2,9 +2,10 @@ ARG GOLANG_VERSION=1.13.8
 ARG DEBIAN_VERSION=10
 
 FROM golang:${GOLANG_VERSION} as builder
+ARG RELEASE_VERSION=
 COPY . /go/src/github.com/tektoncd/cli
 WORKDIR /go/src/github.com/tektoncd/cli
-RUN make bin/tkn
+RUN make RELEASE_VERSION=${RELEASE_VERSION} bin/tkn
 
 FROM debian:${DEBIAN_VERSION} as tkn
 COPY --from=builder /go/src/github.com/tektoncd/cli/bin/tkn /usr/bin