tools for debugging the docker build context

A common complaint about Tilt is that people have trouble figuring out what files are in their Docker context, or how those files map into the container.

For example,

• recognizing that Tilt is ignoring .git (https://github.com/windmilleng/tilt/issues/2169)
• You added a .dockerignore or .tiltignore or only= or ignore=, but forgot
• You want to know (in advance) how changes to a docker_build invocation would change the docker context

A simple first step might be to have a command like:

tilt dump context fe

that dumps all the files in the docker build context for image fe, and creates a tarball you can look through

A more advanced step might be to have a command like:

tilt why path/to/file

(similar to yarn why) that can tell you to how Tilt applies rules for including or excluding a file.

An even more advanced step might be having a web UI over these two commands

Start by reviewing the proposed tilt dump context fe command and the examples involving .dockerignore, .tiltignore, only=, and ignore=. The first step is to narrow the issue to one command and define its output; done should mean the selected Docker context and the rules affecting it are inspectable.