New CI using Github Actions

.github/Dockerfile.CI

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+FROM ubuntu:20.04
+
+# Ubuntu 20.04 timezone fix
+ENV TZ=UTC
+RUN ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone
+
+RUN apt-get update && apt-get install --no-install-recommends -y build-essential libgl1-mesa-dev libsdl2-dev libvorbis-dev libfreetype6-dev mingw-w64
+
+RUN mkdir -p /staging/openarena
+
+COPY . /staging/openarena
+
+RUN cd /staging/openarena && \
+./travis-ci-build.sh

.github/workflows/main.yml

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+name: Build
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v2
+      - name: Build it
+        run:  docker build . -f .github/Dockerfile.CI -t openarena_test