ci: add docker image build

nix/lib.nix

@@ -0,0 +1,9 @@
+{lib, ...}: let
+  firstNonNull = xs: lib.findFirst (x: x != null) "unknown" xs;
+in {
+  gitRevOrLastModified = flake:
+    firstNonNull [
+      (lib.mapNullable (r: "git-" + r) (flake.shortRev or flake.rev or null))
+      (lib.mapNullable (n: "from-" + (lib.toLower n)) flake.sourceInfo.lastModifiedDate or null)
+    ];
+}