Remove Python version from Docker tag names to be published

[toshihikoyanase]

Motivation

This pull request simplifies and refines the Docker image build workflow by removing unnecessary configurations, consolidating Python version handling, and adjusting the Docker push logic. The changes aim to streamline the workflow and improve maintainability.

The Docker tags to be released will be changed as follows:

Current main:

• master push trigger: py3.12
• release trigger: v0.1.0-py3.12 and latest

This PR:

• master push trigger: None
• release trigger: v0.1.0 and latest

Description of the changes

Workflow simplification

• Removed the push trigger for the main branch, leaving only the release and pull_request triggers active to focus on relevant events
• Eliminated the matrix strategy for Python versions, hardcoding the PYTHON_VERSION as an environment variable instead. This simplifies the workflow since only Python 3.12 is supported

Change docker tags

• Updated the Docker build command to use the new PYTHON_VERSION environment variable instead of the matrix variable, ensuring consistency and reducing complexity
• Refined the Docker Hub push logic by removing redundant conditions and assuming a single Python version.