chore: Refactor Dockerfile and build workflow for improved efficiency

- Removed the build stage from the Dockerfile, simplifying the image creation process.
- Updated the Dockerfile to directly copy the pre-built binary instead of using a multi-stage build.
- Modified the build workflow to eliminate unnecessary build arguments, streamlining the configuration.

.gitea/workflows/build.yml

@@ -67,8 +67,6 @@ jobs:
         with:
           context: .
           push: true
-          build-args: |
-            BINARY_NAME=mcauth-linux-amd64
           tags: |
             code.littlelan.cn/carrotskin/mcauth:latest
             code.littlelan.cn/carrotskin/mcauth:${{ github.sha }}