refactor: use version in DOKKU_LIB_ROOT

The version in DOKKU_ROOT may be incorrect for docker-based installs if that directory is mounted from an external volume. Instead, pull it from the DOKKU_LIB_ROOT directory, and respect the STABLE_VERSION if specified (VERSION will be the entire version, including commit sha).

debian/postinst

@@ -88,11 +88,6 @@ setup-sshcommand() {
   fi
 }
 
-setup-version() {
-  rm -f "${DOKKU_ROOT}/VERSION"
-  cp "${DOKKU_LIB_ROOT}/STABLE_VERSION" "${DOKKU_ROOT}/VERSION"
-}
-
 setup-dokku-installer() {
   if [[ -f /etc/nginx/conf.d/dokku-installer.conf ]]; then
     echo "Setting up dokku-installer"
@@ -155,7 +150,6 @@ case "$1" in
     setup-storage
     setup-plugins
     setup-sshcommand
-    setup-version
     setup-dokku-installer
     dpkg-handling
     ;;