devcontainer exec does not always propagate SIGTERM to docker exec

Steps to reproduce:
- Run devcontainer exec without pty, for instance:
```
devcontainer exec --container-id 1234abcdefgh -- sleep infinity > /dev/null 2>&1
```
- Check that on the host machine and inside the container you have infinity sleep process: `ps -auxf |grep "infinity"`
- Interrupt `devcontainer exec` with SIGTERM, i.e. kill the process or just interrupt the terminal.
- Check on the host machine the docker process is gone, but inside the container infinity sleep is dangling.

It seems `docker exec` is terminated without SIGTERM, leading to ungraceful termination.