[dd-ctl] Force DD_DEFAULT_BUILD length to 8 hex digits

This is used to tag Docker images in the registry / pull images form
the registry, it has been observed in the wiled that:

    git rev-parse --short

can have different default values for its length depending on the
system.

We currently specify the length to be 8 as specified here:
https://git-scm.com/docs/git-rev-parse#Documentation/git-rev-parse.txt---shortlength

dd-ctl

@@ -175,7 +175,7 @@ ddupdate(){
 }
 
 build_compose(){
-	DD_DEFAULT_BUILD="$(git rev-parse --short HEAD)"
+	DD_DEFAULT_BUILD="$(git rev-parse --short=8 HEAD)"
 	export DD_BUILD="${DD_BUILD:-${DD_DEFAULT_BUILD}}"
 	setconf DD_BUILD "${DD_BUILD}" .env