Update Dockerfile to Multi-Stage

[multi-stager]

Description

Hi,

Thanks for maintaining sinergym. 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 1615MB to 99MB)
✅ Minimized image vulnerabilities (verified via Trivy, from 698 to 16)

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

Thanks,

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Improvement (of an existing feature)
• Others

Checklist:

• I've read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests.
• I have updated the documentation accordingly.
• I have reformatted the code using autopep8 second level aggressive.
• I have reformatted the code using isort.
• I have ensured cd docs && make spelling && make html pass (required if documentation has been updated.)
• I have ensured pytest tests/ -vv pass. (required).
• I have ensured pytype -d import-error sinergym/ pass. (required)

Changelog:

• Update Dockerfile to Multi-Stage