Add a Dockerfile that matches influxdata-docker to build images from source (#4793)

The images in influxdata-docker are meant to be built by downloading
official releases. Sometimes, it is useful to build directly from source
when you need an unofficial release. These images are meant to be used
then using multi-stage builds so that it can build from source and then
copy the results to images that match the official counterpart.

docker/entrypoint.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+set -e
+
+if [ "${1:0:1}" = '-' ]; then
+    set -- telegraf "$@"
+fi
+
+exec "$@"