Add pip back into the docker image

[Daenara]

After the docker image cleanup quite a few python packages that were installed beforehand are missing (I myself found pytz and requests) and there is no pip installed anymore which makes it nearly impossible to install missing packages.

Rhasspy itself might not need it, but I use a custom command script an spent the last 4 hours searching why everything broke between 2.4.19 and 2.4.20 only to find packages missing.

With the way custom commands work right now I think there should be some way to install packages needed for those commands to work so pip seems essential. Of course it could be replaced by a way to declare what packages are needed that adds them to the docker in another way or by making it possible to let the custom commands run outside of the docker while still using the "old" json dump method.

[synesthesiam]

I can add pip back in (it's removal was unintentional).

Custom commands running outside the Docker image may be more easily implemented as MQTT services for Rhasspy 2.5. I'll add mosquitto_pub and mosquitto_sub to the Docker image too in case you want to rework your custom command as an MQTT send/receive in 2.4.

[Daenara]

I already use an optional mqtt module, I use pip to install paho-mqtt for that right now. I know mqtt is coming in 2.5 but so far I haven't gotten around to playing with 2.5.