Fix the Dockerfile for build

1) Add a .dockerignore file to avoid cache-invalidation by changes in
   .git/ directory.
2) Specify `WORKDIR` once, to eliminates the `cd sysd` command in RUN.
3) Remove the copy action after build to avoid confusion from the
   install instruction in README, since this Dockerfile is just for build.

Signed-off-by: kfei <kfei@kfei.net>
Merge pull request #46 from hacking-thursday/dockerfile-for-build

Fix the Dockerfile for build