Add missing zstd-libs to final Docker image

Also add `minetestserver --version` command to verify docker build in CI

.github/workflows/build.yml

@@ -180,7 +180,8 @@ jobs:
       - uses: actions/checkout@v2
       - name: Build docker image
         run: |
-          docker build .
+          docker build . -t minetest:latest
+          docker run --rm minetest:latest /usr/local/bin/minetestserver --version
 
   win32:
     name: "MinGW cross-compiler (32-bit)"