Refactor Dockerfile to use multi-stage builds and add caching mechanisms

[catatsuy]

This pull request includes significant updates to the Dockerfile for the Golang web application. The changes improve build performance, add multi-stage builds, and enhance security by running the application as a non-privileged user.

Dockerfile improvements:

• Changed the Dockerfile syntax to use docker/dockerfile:1 for enhanced features and performance.
• Implemented multi-stage builds to separate the build environment from the runtime environment, improving security and reducing the final image size.
• Added --mount options for caching dependencies and binding source files, which speeds up the build process.
• Created a non-privileged user to run the application, following best practices for container security.
• Switched the base image for the runtime environment to alpine:3.20 for a smaller, more secure final image.