You shouldn’t have to access the container. If you can access the file system on the machine that is running Rhasspy, go to your profile directory, as per the docs:
This will start Rhasspy with the English profile (en) in the background (-d) on port 12101 (-p) and give Rhasspy access to your microphone (--device). Any changes you make to your profile will be saved to /home/<YOUR_USER>/.config/rhasspy.
Change to the language profile directory (if you’re using english: /home/<YOUR_USER>/.config/rhasspy/en) and create a new directory for your sound file/s (maybe name it wav to keep things simple)
Upload your new wav files to:
/home/<YOUR_USER>/.config/rhasspy/en/wav
Then in the “Sounds” section of Settings in Rhasspy, change your “Wake” Wav section to:
${RHASSPY_PROFILE_DIR}}/wav/YourNewWakeWordSound.wav