You seem to already know the answer to this, set up distcc to use SSH. This will ensure encrypted communication between your distcc client and the distcc servers you have deployed as Docker images in the cloud. You have highlighted that the cost of doing this would be spending time to set up an SSH key that would be accepted by all of your Docker images. From memory this key could be the same for all the Docker nodes, as long as they all had the same user name using the same key. Is that really such a complex task?
You ask for a slightly less secure option for building your Compile Farm. Well limiting things based on the Internet accessible IP address for your house would limit the scope and increase the complexity of others using your build cluster. Someone might spoof the special IP address and get access to your distcc servers but that would just cost you their runtime. The larger concern would be that your code could be transmitted in plain text over the internet to these distcc servers. If that is not a big concern then it could be considered low risk.
An alternative might be to setup a secure remote network of docker nodes and set up VPN access to them. This would bind your local machine to the remote network and you could consider the whole thing to be a secured LAN. If it is considered safe to have the Docker nodes talk between themselves in an unencrypted manner within the cloud, it should be as secure to have a VPN link to them and do the same.
They best option might be to dig out a some old PCs and set those up as local distcc servers. Within a LAN their is no need for security.
You mention a wish to share this with other Raspberry PI users. There have been other Public Compile Farms in the past but many of them have fallen out of favour. Distributing such things publicly, as computational projects such as BOINC do, works poorly because the network latency and transfer rates can slow the builds significantly.
