Dockerfile: update to latest syntax

It was pinned to the 1.3 version; removing the minor version to
make sure we're on the latest 1.x stable.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.3
+# syntax=docker/dockerfile:1
 
 ARG CROSS="false"
 ARG SYSTEMD="false"