Instead of using Nix to build whatsxmpp, we switch back to using Docker in order
to make use of daewok's SBCL static executables fork
(ref: https://www.timmons.dev/posts/static-executables-with-sbcl-v2.html).
The Dockerfile, adapted from the sources of his fork, pulls his fork, builds
SBCL with sb-prelink-linkage-table, and then does the necessary stuff to make
a fully static whatsxmpp binary.
Importantly, the new binary uses SB-EXT:RESTRICT-COMPILER-POLICY to set 'safety
to 3 in all packages, to hopefully mitigate the heap corruption issues some
users are seeing. (The fact that Nix couldn't do this was a key reason for the
move away.)
We remove the old Nix files to avoid cruft / confusion.
