Added GitHub CodeSpaces development environment.

.devcontainer/GZDoom.dockerfile

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+FROM ubuntu:latest
+LABEL org.opencontainers.image.authors="CandiceJoy <[email protected]>"
+LABEL author="CandiceJoy"
+LABEL description="GZDoom compilation image (Designed for GZDoom 4.11pre)"
+LABEL verion="4.11pre"
+
+# Update these as needed
+ENV GZ_ZMUSIC_URL="https://github.com/coelckers/ZMusic.git"
+ENV GZ_ZMUSIC_COMMIT="75d2994b4b1fd6891b20819375075a2976ee34de"
+ENV GZ_PACKAGES="build-essential git cmake libsdl2-dev libvpx-dev"
+
+# Update package lists and install package-based build dependencies
+RUN apt-get update; apt-get install -y $GZ_PACKAGES
+
+# Install ZMusic
+RUN git clone $GZ_ZMUSIC_URL; git reset --hard $GZ_ZMUSIC_COMMIT; cd ZMusic; cmake . -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr; make; make install

.devcontainer/devcontainer.json

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+{
+  "build":{ "dockerfile": "GZDoom.dockerfile" },
+  "name": "GZDoom",
+  "features": {
+  }
+}