fix: align Docker Go version with go.mod and optimize logger

[hobostay]

This PR fixes build consistency issues and optimizes the logger for better memory efficiency.

Changes

Dockerfile

• Updated base image from golang:1.26.0-alpine to golang:1.25-alpine
• Aligns with go.mod which specifies go 1.25.7
• Prevents potential build failures due to version mismatch

pkg/logger/logger.go

• Optimized file write operation by using append(jsonData, '\n') instead of string(jsonData) + "\n"
• This avoids an unnecessary string allocation and conversion
• Added explicit empty string assignment for fieldStr when no fields are present
• Important for the project's goal of running on devices with <10MB RAM

Benefits

1. Build consistency: Docker build now uses the same Go version as go.mod
2. Memory efficiency: Reduced allocations in the hot logging path
3. Performance: Single Write call instead of WriteString with string concatenation

Testing

• No functional changes to logging behavior
• All log output remains identical
• Memory allocations reduced in log path

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

[xiaket]

Can you please update this PR to use the default PR template?