Fix caching docker build

For some as-yet-not-investigated reason, the double-build technique that enables docker caching, currently commented-out in the Dockerfile, causes the docker build to fail thusly:

   Compiling actix-files v0.1.4
   Compiling kvdbd v0.3.1 (/usr/src/kvdbd)
error: couldn't read src/kvdb-pb.rs: No such file or directory (os error 2)

error: aborting due to previous error

error: Could not compile `kvdbd`.
warning: build failed, waiting for other jobs to finish...
error: couldn't read src/tester.rs: No such file or directory (os error 2)

error: aborting due to previous error

error: Could not compile `kvdbd`.
warning: build failed, waiting for other jobs to finish...
error: build failed
The command '/bin/sh -c cargo build --release' returned a non-zero code: 101