Exec: Add ability to set environment variables

Keeping the current behavior for exec, i.e., inheriting
variables from main process. New variables will be added
to current ones. If there's already a variable with that
name it will be overwritten.

Example of usage: docker exec -it -e TERM=vt100 <container> top

Closes #24355.

Signed-off-by: Jonh Wendell <jonh.wendell@redhat.com>

docs/reference/commandline/exec.md

@@ -14,6 +14,7 @@ Run a command in a running container
 Options:
   -d, --detach         Detached mode: run command in the background
       --detach-keys    Override the key sequence for detaching a container
+  -e, --env=[]         Set environment variables
       --help           Print usage
   -i, --interactive    Keep STDIN open even if not attached
       --privileged     Give extended privileges to the command