Open .dockerignore as utf-8

[carlbordum]

This should solve docker/compose#7236

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "dockerignore-encoding" git@github.com:carlbordum/docker-py.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[milas]

We now only support Python3 and the file will be opened using the default system encoding:

The default encoding is platform dependent (whatever locale.getpreferredencoding() returns)

https://docs.python.org/3/library/functions.html#open

That said, I think that .dockerignore files are likely implicitly UTF-8 due to the Docker tooling being primarily written in Go, in which strings are kinda sorta UTF-8 if you don't think too hard. So, I'd be willing to accept an updated PR for Python3 for this if it's still causing someone trouble.

[carlbordum]

I don't think this is a Python 2/3 issue, but an issue because Windows default encoding is not UTF-8.