add github actions

.github/FUNDING.yml

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+github: hackebein
+custom: paypal.me/hackebein

.github/workflows/build.yml

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+name: Build
+on:
+  push:
+    branches:
+    - master
+  schedule:
+  - cron: 0 4 * * 1
+jobs:
+  latest:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up QEMU
+      uses: docker/setup-qemu-action@v1
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v1
+    - name: Login to DockerHub
+      uses: docker/login-action@v1
+      with:
+        username: ${{ secrets.DOCKERHUB_USERNAME }}
+        password: ${{ secrets.DOCKERHUB_TOKEN }}
+    - name: Build and push
+      uses: docker/build-push-action@v2
+      with:
+        push: 'true'
+        platforms: linux/amd64
+        tags: hackebein/garrysmod:latest