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>
Upstream-commit: e03bf1221ee2c863f25a57af4d415e2d8ff4f26c
Component: engine

components/engine/libcontainerd/client_linux.go

@@ -45,7 +45,7 @@ func (clnt *client) AddProcess(ctx context.Context, containerID, processFriendly
 	sp := spec.Process
 	sp.Args = specp.Args
 	sp.Terminal = specp.Terminal
-	if specp.Env != nil {
+	if len(specp.Env) > 0 {
 		sp.Env = specp.Env
 	}
 	if specp.Cwd != nil {