Switch Docker to Alpine and build kge.

Update build environment to Alpine, enable GUI support, and refine developer guide

- Migrated Dockerfile base image from Ubuntu 22.04 to Alpine 3.19 for a smaller and faster container.
- Added dependencies for GUI support (SDL2, OpenGL/Mesa, Freetype, etc.) and updated CMake options.
- Enhanced `DEVELOPER_GUIDE.md` with new instructions for GUI builds, updated dependencies, and simplified custom build workflows.
- Addressed Alpine-specific ncurses library path issues in CMake configuration.

Dockerfile

@@ -1,20 +1,23 @@
 # Minimal Dockerfile for building and testing kte on Linux
 # This container provides a build environment with all dependencies.
 # Mount the source tree at /kte when running the container.
-FROM ubuntu:22.04
-
-# Avoid interactive prompts during package installation
-ENV DEBIAN_FRONTEND=noninteractive
+FROM alpine:3.19
 
 # Install build dependencies
-RUN apt-get update && apt-get install -y \
-    build-essential \
+RUN apk add --no-cache \
+    g++ \
     cmake \
-    libncursesw5-dev \
-    && rm -rf /var/lib/apt/lists/*
+    make \
+    ncurses-dev \
+    sdl2-dev \
+    mesa-dev \
+    freetype-dev \
+    libx11-dev \
+    libxext-dev
 
 # Set working directory where source will be mounted
 WORKDIR /kte
 
 # Default command: build and run tests
-CMD ["sh", "-c", "cmake -B build -DBUILD_GUI=OFF -DBUILD_TESTS=ON && cmake --build build --target kte && cmake --build build --target kte_tests && ./build/kte_tests"]
+# Add DirectFB include path for SDL2 compatibility on Alpine
+CMD ["sh", "-c", "cmake -B build -DBUILD_GUI=ON -DBUILD_TESTS=ON -DCMAKE_CXX_FLAGS='-I/usr/include/directfb' && cmake --build build --target kte && cmake --build build --target kge && cmake --build build --target kte_tests && ./build/kte_tests"]