Fix Docker issues

[castholm]

Thanks for this really useful tool. I ran into a couple of minor issues on Windows when following the instructions for building a docker image:

• If your git isn't configured to preserve line endings (meaning core.autocrlf is set to true), bootstrap and .ac/.am files will be checked out with CRLF line endings resulting in the shell complaining about files not existing. This is fixed by enforcing LF line endings for these files in .gitattributes, which makes checking out "just work" regardless of the user's settings.
• If you run docker build after cloning the repo but before initializing git submodules, Docker will try to initialize submodules but fail with several messages about failing to validate server certificates. Installing the ca-certificates package fixes this.