add multiple env-file

Signed-off-by: jason yang <[email protected]>

environment_and_metadata.rst

@@ -273,17 +273,28 @@ variables include special characters.
 ``--env-file`` option
 =====================
 
-The ``--env-file`` option lets you provide a file that contains
+The ``--env-file`` option lets you provide multiple files that contain
 environment variables as ``NAME=VALUE`` pairs, e.g.:
 
 .. code::
 
    $ cat myenvs
    MYVAR="Hello from a file"
 
-   $ {command} run --env-file myenvs env.sif
+   $ cat myenvs2
+   MYVAR="Hello from a file 2"
+
+   $ {command} run --env-file myenvs --env-file myenvs2 env.sif
+   Hello from a file 2
+
+   $ {command} run --env-file myenvs2,myenvs env.sif
    Hello from a file
 
+.. note::
+
+   Note that environment variables presented in the ``--env-file`` file will 
+   always overwrite the same variables defined in the former file.
+
 ``{ENVPREFIX}ENV_`` prefix
 ==========================