Automatically build Dockerfiles

• Omit from from the binci.yml to automatically build and execute tasks against a Dockerfile in the local project
• Use the new dockerfile parameter to specify an alternate dockerfile to build for binci task execution. Example: dockerfile: ./Dockerfile.dev
• Use the work workDir parameter to specify where the current working directory should be mounted in the main docker container, which will also match the working directory path. Omitting this will cause the folder to be mounted under the same absolute path inside the container as always. However, if your Dockerfile copies your app to /app, it may be convenient to specify workDir: /app to keep your files in the expected places.
• New images are automatically built if the Dockerfile changes. Binci will clean up after itself by deleting the most recent previously-built image every time it builds a new one. If the Dockerfile hasn't changed since the last run, Binci won't build it again.
• The CLI now supports a -b parameter to specify a different Dockerfile to build for this task, overriding any from or dockerfile setting in the binci.yml. Similarly, specifying a new "from" base image with -f will cause tasks to run against that image even if binci.yml is configured to build a Dockerfile.