Allow exclude section to point at existing ignore file

[nickhammond]

Description

Most container-based applications already have a .dockerignore and a .gitignore file, it'd be great to be able to point to one of these files for the exclude section of your project descriptor.

Proposed solution

1. Add a file-based reference to the exclude section, something like:

exclude_list = ".dockerignore"

2. Potentially add a CLI option --exclude_list .dockerignore

Either option would read .dockerignore and utilize that as the exclude option for the pack instead of listing them out.

Describe alternatives you've considered

1. Open .dockerignore
2. Copy entire file contents
3. Create project.toml with base spec and an exclude section
4. Within the exclude section paste in the lines from your .dockerignore file
5. Remove all comments and empty lines
6. Quote each line and turn into a proper array of directives so that it looks something like this:

[io.buildpacks]
exclude = [
  "/.git/",
  ".env"
]

9. Remember to maintain two ignore/exclude lists, the one in project.toml and the other one in .dockerignore

Additional context

• project.toml docs