fix: resolve Docker entrypoint error on Windows by enforcing LF line endings for shell scripts

[billxc]

What issue this PR fixes

This PR resolves a Docker entrypoint error that occurs when building and running the container on Windows. The error is caused by Windows-style (CRLF) line endings in entrypoint.sh, which prevent the shell from executing the script correctly in the Linux-based container.

How to reproduce the issue

1. Clone the repository on a Windows machine.
2. Build the Docker image:

   docker build -t copilot-api .

3. Run the container:

   docker run copilot-api

4. You will see an error similar to:

   exec /entrypoint.sh: no such file or directory
   

Root cause

When shell scripts are created or edited on Windows, they default to CRLF (Carriage Return + Line Feed, \r\n) line endings. When the shebang line #!/bin/sh contains a \r character, the Linux kernel interprets it as part of the interpreter path (/bin/sh\r), which doesn't exist. This causes the "no such file or directory" error even though the file itself is present in the container.

Solution

This PR adds a .gitattributes file to enforce LF line endings for shell scripts, ensuring compatibility with Docker on Windows.