fix: ensure we respect DOKKU_DEPLOY_BRANCH when rebuilding applications

Closes #3021

plugins/git/functions

@@ -145,7 +145,8 @@ git_build() {
   if [[ $# -ge 2 ]]; then
     local REF="$2"
   else
-    local REF=$(< "$DOKKU_ROOT/$APP/refs/heads/master")
+    DOKKU_DEPLOY_BRANCH="$(git_deploy_branch "$APP")"
+    local REF=$(< "$DOKKU_ROOT/$APP/refs/heads/$DOKKU_DEPLOY_BRANCH")
   fi
 
   # shellcheck disable=SC2086

tests/unit/20_config.bats

@@ -153,9 +153,19 @@ teardown() {
   echo "status: "$status
   assert_success
 
+  run ssh dokku@dokku.me ps:rebuild $TEST_APP
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
   run ssh dokku@dokku.me config:set $TEST_APP DOKKU_DEPLOY_BRANCH=app-branch
   GIT_REMOTE_BRANCH=app-branch deploy_app
   echo "output: "$output
   echo "status: "$status
   assert_success
+
+  run ssh dokku@dokku.me ps:rebuild $TEST_APP
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
 }