[FrameworkBundle] Support APP_BUILD_DIR

Author: alexandre-daubois

reference/configuration/kernel.rst

@@ -22,6 +22,14 @@ container or AWS Lambda.
 This value is also exposed via the :method:`Symfony\\Component\\HttpKernel\\Kernel::getBuildDir`
 method of the kernel class, which you can override to return a different value.
 
+You can also change the build directory by defining an environment variable
+named ``APP_BUILD_DIR`` whose value is the full path of the build folder.
+
+.. versionadded:: 6.4
+
+    The support of the ``APP_BUILD_DIR`` environment variable was introduced in
+    Symfony 6.4.
+
 ``kernel.bundles``
 ------------------