Build on older ubuntu

Author: rnijveld

.github/workflows/docker.yml

@@ -46,7 +46,7 @@ jobs:
             docker_arch: arm64
           - rust_arch: x86_64-unknown-linux-gnu
             docker_arch: amd64
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Setup Rust toolchain
         uses: dtolnay/rust-toolchain@a54c7afa936fefeb4456b2dd8068152669aa8203