Add stead sdk docker file to build for steam.

docker/Dockerfile.steam

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+# Build:        docker build -t paintown-steam-build -f docker/Dockerfile.steam .
+# Access shell: docker run --rm -it --entrypoint /bin/bash  paintown-steam-build 
+# Use sniper build (https://github.com/ValveSoftware/steam-runtime?tab=readme-ov-file#installation)
+FROM registry.gitlab.steamos.cloud/steamrt/sniper/sdk:latest
+
+RUN apt update && \ 
+    apt install -y sudo wget git ninja-build make python3-pip libsdl2-gfx-dev && \
+    apt remove -y meson
+
+# Get latest meson
+RUN pip install meson && \
+    ln -s /usr/local/bin/meson /usr/bin/meson
+
+# Build dirs
+RUN mkdir -p /build/cross/include/SDL2
+
+COPY src /build/src
+COPY meson.build /build
+COPY misc/steam/steam.txt /build/steam.txt
+COPY meson.options /build
+COPY Makefile /build/Makefile
+
+# Makefile for cross in case we need to do something different
+RUN echo ".PHONY: all clean\n\
+\n\
+steam: build-steam\n\
+\t(cd build-steam; meson configure -Dbuild_tests=false)\n\
+\tmeson compile -C build-steam\n\
+\n\
+build-steam:\n\
+\tmkdir build-steam\n\
+\tmeson setup --cross-file steam.txt build-steam\n\
+\n\
+clean:\n\
+\trm -fr build-steam" > /build/Makefile-steam
+
+# Temp directory
+RUN mkdir -p /build/.tmp
+
+# SDL_gfx is not installed by default, we may need to statically link it if it doesn't work on steam deck or with steam natively
+#RUN git clone https://github.com/ferzkopp/SDL2_gfx.git -- /build/.tmp/SDL_gfx
+#RUN echo "sources = [\n\
+#    'SDL2_framerate.c',\n\
+#    'SDL2_gfxPrimitives.c',\n\
+#    'SDL2_imageFilter.c',\n\  
+#    'SDL2_rotozoom.c',\n\
+#]\n\
+#\n\
+#includes = include_directories('.')\n\
+#\n\
+#sdl_graphics_library = static_library('SDL2_gfx', sources,\n\
+#    include_directories: [includes],\n\
+#    dependencies: [sdl_dependency, zlib_dependency],\n\
+#)\n" > /build/.tmp/SDL_gfx/meson.build
+#RUN cp /build/.tmp/SDL_gfx/*.h /build/cross/include/SDL2
+
+RUN cd /build && make
+
+CMD [ "/bin/bash" ]
+

misc/steam/steam.txt

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+[constants]
+cross_root = 'cross'
+common_args = ['-I..' / cross_root / 'include',
+               '-DDATA_PATH="data"',
+               '-DSTEAM'
+              ]
+#common_link_args = ['-L/usr/x86_64-w64-mingw32/lib', '-lz']
+
+[binaries]
+c = 'clang'
+cpp = 'clang++'
+ar = 'ar'
+strip = 'strip'
+#exe_wrapper = 'wine64'
+pkg-config = 'pkg-config'
+sdl2-config = 'sdl2-config'
+cmake = 'cmake'
+
+[properties]
+#pkg_config_libdir = ['/usr/x86_64-w64-mingw32/lib/pkgconfig', cross_root / 'lib/pkgconfig']
+pkg_config_libdir = []
+
+[host_machine]
+system = 'steamos'
+cpu_family = 'x86_64'
+cpu = 'x86_64'
+endian = 'little'
+
+[built-in options]
+#pkg_config_path = cross_root / 'lib/pkg-config'
+#c_include_path = cross_root / 'include'
+#cpp_include_path = cross_root / 'include'
+c_args = common_args
+cpp_args = common_args
+c_link_args = ['-static', '-lsfml-window-s', '-lsfml-system-s',]
+cpp_link_args = ['-static', '-static-libgcc', '-static-libstdc++',]
+#includedir = cross_root / 'include'
+#libdir = cross_root / 'lib'
+default_library='static'
+
+[sdl2:built-in options]
+default_library = 'static'
+werror = false
+
+[SDL2_image:built-in options]
+default_library = 'static'
+werror = false
+
+[SDL2_ttf:built-in options]
+default_library = 'static'
+werror = false
+
+[SDL2_mixer:built-in options]
+default_library = 'static'
+werror = false
+
+[zlib:built-in options]
+default_library = 'static'
+werror = false
+
+[yaml-cpp:built-in options]
+default_library = 'static'
+werror = false