refactor: remove need for internal dokku calls

Avoid internal dokku calls will avoid potentially expensive subprocesses - in particular, user-auth will be re-invoked, which can be expensive.

plugins/common/functions

@@ -17,6 +17,17 @@ dokku_apps() {
   [[ $INSTALLED_APPS ]] && echo "$INSTALLED_APPS"
 }
 
+dokku_version() {
+  if [[ -f "$DOKKU_LIB_ROOT/STABLE_VERSION" ]]; then
+    DOKKU_VERSION=$(cat "${DOKKU_LIB_ROOT}/STABLE_VERSION")
+  elif [[ -f "$DOKKU_LIB_ROOT/VERSION" ]]; then
+    DOKKU_VERSION=$(cat "${DOKKU_LIB_ROOT}/VERSION")
+  else
+    dokku_log_fail "Unable to determine dokku's version"
+  fi
+  echo "dokku version ${DOKKU_VERSION}"
+}
+
 dokku_log_quiet() {
   declare desc="log quiet formatter"
   if [[ -z "$DOKKU_QUIET_OUTPUT" ]]; then