Support for dev image about docker-sia

[ro-tex]

So, we need a second container, alongside the main production container that the portal is going to be using. This second container should have the dev tools needed to build siad and the source code. This part is easy and straightforward.

The debug container should be able to build siac and siad binaries which, crucially, should be used by the main production container after it's restarted. No problem there but there are some considerations.

The way to do it:

• create a new dir on the server and mount it as a volume on both the production and debug containers
• add this volume to the PATH of the production container before /usr/bin, so when the container is restarted and tries to execute siad from the PATH it will run the version from the volume
• from the debug container we can build siac and siad and copy them to the volume and then restart the production container

Considerations:

• the production container will be blindly running a binary that's provided to it via a volume and this is a security consideration. I'm not sure how severe that is because anyone who can place a binary there should also be able to control docker.
• the process is quite clunky and doesn't allow the developer to locally test the binary before sending it to the production container, unless we keep a second copy of the consensus to be used from the debug container.
• if we use a second copy of the consensus, maybe we can copy the consensus mounted on the production image, strip all wallet-related data and reinitialize it for the new node. I could need some guidance here in order to not miss anything.

[ro-tex]

@kwypchlo your proposal can definitely work. I actually like it more than mine. Let's do that.