Update Dockerfile to Multi-Stage

[multi-stager]

Hi,

Thanks for maintaining plaid-python. I represent a research group investigating multi-stage builds. We recently refactored your Dockerfile to a multi-stage Dockerfile and found that it brings the following benefits:

✅ Reduced final image size (from 1281MB to 972MB)
✅ Minimized image vulnerabilities (verified via Trivy, from 7 to 3)

We hope this minor improvement to the Dockerfile will be useful to you :)

Thanks,