Fix setting the dataplaneapi version in the haproxy docker build.

The ARG that sets the version string in the Dockerfile must evidently
be set *after* the FROM statement, so that git checkout is invoked
correctly.

container/Dockerfile.haproxy

 ARG go_version=1.17
 ARG debian_tag=11.1-slim
-ARG dataplane_version=v2.3.1
 
 # Build k8s-crt-dnldr in the builder stage
 FROM golang:${go_version} as builder
 
+ARG dataplane_version=v2.3.1
+
 RUN apt-get update -q && \
     apt-get install -y golang-golang-x-tools pandoc
 
@@ -16,11 +17,8 @@ RUN go get -d -v github.com/slimhazard/gogitversion && \
 RUN cd /go && git clone https://code.uplex.de/k8s/k8s-crt-dnldr.git && \
     cd /go/k8s-crt-dnldr && make k8s-crt-dnldr
 
-# RUN cd /go && git clone https://github.com/haproxytech/dataplaneapi.git && \
-#    cd /go/dataplaneapi && git checkout ${dataplane_version} && make build
-
 RUN cd /go && git clone https://github.com/haproxytech/dataplaneapi.git && \
-    cd /go/dataplaneapi && git checkout v2.3.1 && make build
+    cd /go/dataplaneapi && git checkout ${dataplane_version} && make build
 
 # Build haproxy
 FROM debian:${debian_tag}