Create Docker image with cryptol-remote-api as the entrypoint

[atomb]

Create a variation of the main Cryptol Dockerfile that builds cryptol-remote-api instead of cryptol and makes it the entrypoint. Once the HTTP transport is complete, have the entrypoint enable that mode by default.

[atomb]

A first draft of this Dockerfile exists as of commit 2919e21. The next step is to finish the HTTP transport and tweak the entrypoint to pass in the flag to enable HTTP.

[atomb]

This is now automatically built, with the HTTP mode as the default, as of #967.