Dockerfile: switch to "stable" dockerfile front-end

The `RUN --mount` options have been promoted to the stable channel,
so we can switch from "experimental" to "stable".

Note that the syntax directive should no longer be needed now, but
it's good practice to add a syntax-directive, to allow building on
older versions of docker.

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

Dockerfile

@@ -1,4 +1,4 @@
-# syntax=docker/dockerfile:1.1.7-experimental
+# syntax=docker/dockerfile:1.2
 
 ARG CROSS="false"
 ARG SYSTEMD="false"