feat: add custom godot-ci export-toolchain image

Bakes export-template.yml's apt-get toolchain into an image extending
barichello/godot-ci:4.7.1, plus a publish workflow to push it to this
repo's Gitea package registry.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,12 @@
+FROM barichello/godot-ci:4.7.1
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libfontconfig1 \
+    build-essential scons pkg-config xz-utils curl \
+    libx11-dev libxcursor-dev libxinerama-dev libgl1-mesa-dev \
+    libasound2-dev libpulse-dev libudev-dev libxi-dev libxrandr-dev libwayland-dev \
+    mingw-w64 \
+    blender \
+  && update-alternatives --set x86_64-w64-mingw32-gcc /usr/bin/x86_64-w64-mingw32-gcc-posix \
+  && update-alternatives --set x86_64-w64-mingw32-g++ /usr/bin/x86_64-w64-mingw32-g++-posix \
+  && rm -rf /var/lib/apt/lists/*