Build our own HAproxy docker image

The HAproxy docker image we were using is no longer maintained (hasn't been
for years), but is built upon Alpine Linux which has a big security leak:
https://talosintelligence.com/vulnerability_reports/TALOS-2019-0782

The security leak is fixed in this build of the docker image, but we should
move to something else (lke Træfik).

docker/haproxy/build.sh

@@ -0,0 +1,10 @@
+#!/bin/bash -e
+
+# When updating this, also update the version in Dockerfile
+VERSION=1.6.7
+
+docker build -t armadillica/haproxy:${VERSION} .
+docker tag armadillica/haproxy:${VERSION} armadillica/haproxy:latest
+
+echo "Done, built armadillica/haproxy:${VERSION}"
+echo "Also tagged as armadillica/haproxy:latest"