dockerTools.streamLayeredImage: resolve duplicate env vars

For images running on Kubernetes, there is no guarantee on how duplicate
environment variables in the image config will be handled. This seems
to be different from Docker, where the last environment variable value
is consistently selected.

The current code for `streamLayeredImage` was exploiting that assumption
to easily propagate environment variables from the base image, leaving
duplicates unchecked. It should rather resolve these duplicates to
ensure consistent behavior on Docker and Kubernetes.

nixos/tests/docker-tools.nix

@@ -221,6 +221,21 @@ import ./make-test-python.nix ({ pkgs, ... }: {
         assert "FROM_CHILD=true" in env, "envvars from the child should be preserved"
         assert "LAST_LAYER=child" in env, "envvars from the child should take priority"
 
+    with subtest(
+        "Ensure inherited environment variables of layered images are correctly resolved"
+    ):
+        # Read environment variables as stored in image config
+        config = docker.succeed(
+            "tar -xOf ${examples.environmentVariablesLayered} manifest.json | ${pkgs.jq}/bin/jq -r .[].Config"
+        ).strip()
+        out = docker.succeed(
+            f"tar -xOf ${examples.environmentVariablesLayered} {config} | ${pkgs.jq}/bin/jq -r '.config.Env | .[]'"
+        )
+        env = out.splitlines()
+        assert (
+            sum(entry.startswith("LAST_LAYER") for entry in env) == 1
+        ), "envvars overridden by child should be unique"
+
     with subtest("Ensure image with only 2 layers can be loaded"):
         docker.succeed(
             "docker load --input='${examples.two-layered-image}'"

pkgs/build-support/docker/stream_layered_image.py

@@ -202,7 +202,9 @@ def overlay_base_config(from_image, final_config):
     # Preserve environment from base image
     final_env = base_config.get("Env", []) + final_config.get("Env", [])
     if final_env:
-        final_config["Env"] = final_env
+        # Resolve duplicates (last one wins) and format back as list
+        resolved_env = {entry.split("=", 1)[0]: entry for entry in final_env}
+        final_config["Env"] = list(resolved_env.values())
     return final_config