tests/nixos: add more thorough nix-docker tests

tests/nixos/nix-docker-test.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+# docker.nix test script. Runs inside a built docker.nix container.
+
+set -eEuo pipefail
+
+export NIX_CONFIG='substituters = http://cache:5000?trusted=1'
+
+cd /tmp
+
+# Test getting a fetched derivation
+test "$("$(nix-build -E '(import <nixpkgs> {}).hello')"/bin/hello)" = "Hello, world!"
+
+# Test building a simple derivation
+# shellcheck disable=SC2016
+nix-build -E '
+let
+  pkgs = import <nixpkgs> {};
+in
+builtins.derivation {
+  name = "test";
+  system = builtins.currentSystem;
+  builder = "${pkgs.bash}/bin/bash";
+  args = ["-c" "echo OK > $out"];
+}'
+test "$(cat result)" = OK
+
+# Ensure #!/bin/sh shebang works
+echo '#!/bin/sh' > ./shebang-test
+echo 'echo OK' >> ./shebang-test
+chmod +x ./shebang-test
+test "$(./shebang-test)" = OK
+
+# Ensure #!/usr/bin/env shebang works
+echo '#!/usr/bin/env bash' > ./shebang-test
+echo 'echo OK' >> ./shebang-test
+chmod +x ./shebang-test
+test "$(./shebang-test)" = OK
+
+# Test nix-shell
+{
+    echo '#!/usr/bin/env nix-shell'
+    echo '#! nix-shell -i bash'
+    echo '#! nix-shell -p hello'
+    echo 'hello'
+} > ./nix-shell-test
+chmod +x ./nix-shell-test
+test "$(./nix-shell-test)" = "Hello, world!"