Add support for character classes

[kf6kjg]

Or more generally: support what gitignore supports.

As of right now, .dockerignore files with changes such as github/gitignore@7b35656 will cause the engine to choke and die with an error like ERROR: failed to solve: Internal: rpc error: code = Internal desc = rpc error: code = Internal desc = header key "exclude-patterns" contains value with non-printable ASCII characters.

My recommendation: move away from DIY ignorefile parsing and use an existing lib, maybe even a part of go-git's implementation. I'd personally recommend a lib instead of a source code copy, but they are Apache 2 as well, so there shouldn't be any license issue from code copying - just a maintenance burden, but one I'd argue is less than the current maintenance burden.

This kind of change might be a breaking change for some users: matching outcomes may change for

• malformed/ambiguous patterns,
• path normalization differences (filepath.Clean, separators),
• bracket/escape semantics,
• negation ordering edge cases.

However I'd argue that this is worth it:

1. users can use standard snippets, including the same blocks they use in their other .*ignore files,
2. users that complain about the structure or feature support of the file format can be pointed at git's dev team. :)