Improve dockerfile build time

[ibnesayeed]

A Dockerfile was added as per #342 which generates a minimal image, but each time an image is built, all the dependencies are downloaded again because of the cache invalidation. This can be improved by some strategic placement of instructions in the Dockerfile and resolving dependencies before coping the whole source code in.

Another possibility of improvement is exclusion of unnecessary stuff from Docker's build context by including a .dockerignore file.