Fossil SCM

Switched to a split ENTRYPOINT/CMD scheme for launching the Fossil server in the container. The immediate need is so we lower-level ENTRYPOINTs provided by mix-in layers, but it's more correct generally. ENTRYPOINT says this is the hard-coded purpose of the container, and CMD gives the arguments to that command. The split is therefore between the mandatory parts and the parts the user might want to override without needing to write their own Dockerfile.