ci: only build docker on non-arm architectures

ARM based runners do not have KVM which is required for `runAsRoot` for
`dockerTools`.

Author: CompeyDev

.github/workflows/ci.yml

@@ -68,4 +68,5 @@ jobs:
         run: nix build --file . ssh-portfolio${CARGO_FEATURES:+-$CARGO_FEATURES} 
 
       - name: Build docker image
+        if: ${{ runner.arch }} != "ARM64"
         run: nix build .#docker