Use .dockerignore to pass --exclude parameters to the context tar file creation

A "git" like ignore file that uses the existing Excludes config option from archive to filter out files that are not needed in the container context. 

Useful for:
- excluding private/confidential/config files you may not wish to push to the public registry
- excluding source control, editor files and other workflow related files
- excluding logs and temporary files

Lightly tested for our specific use case, but as long as their lines are valid for `tar` it will work fine. Even does pattern matching if you are in to that sort of thing.

As discussed in issue #2224.