dockerTools: Always set imageTag attribute

The image tag can be specified or generated from the output hash.
Previously, a generated tag could be recovered from the evaluated
image with some string operations.

However, with the introduction of streamLayeredImage, it's not
feasible to compute the generated tag yourself.

With this change, the imageTag attribute is set unconditionally,
for the buildImage, buildLayeredImage, streamLayeredImage functions.

nixos/tests/docker-tools.nix

@@ -30,8 +30,45 @@ import ./make-test-python.nix ({ pkgs, ... }: {
         )
 
     docker.succeed("docker run --rm ${examples.bash.imageName} bash --version")
+    # Check imageTag attribute matches image
+    docker.succeed("docker images --format '{{.Tag}}' | grep -F '${examples.bash.imageTag}'")
     docker.succeed("docker rmi ${examples.bash.imageName}")
 
+    # The remaining combinations
+    with subtest("Ensure imageTag attribute matches image"):
+        docker.succeed(
+            "docker load --input='${examples.bashNoTag}'"
+        )
+        docker.succeed(
+            "docker images --format '{{.Tag}}' | grep -F '${examples.bashNoTag.imageTag}'"
+        )
+        docker.succeed("docker rmi ${examples.bashNoTag.imageName}:${examples.bashNoTag.imageTag}")
+
+        docker.succeed(
+            "docker load --input='${examples.bashNoTagLayered}'"
+        )
+        docker.succeed(
+            "docker images --format '{{.Tag}}' | grep -F '${examples.bashNoTagLayered.imageTag}'"
+        )
+        docker.succeed("docker rmi ${examples.bashNoTagLayered.imageName}:${examples.bashNoTagLayered.imageTag}")
+
+        docker.succeed(
+            "${examples.bashNoTagStreamLayered} | docker load"
+        )
+        docker.succeed(
+            "docker images --format '{{.Tag}}' | grep -F '${examples.bashNoTagStreamLayered.imageTag}'"
+        )
+        docker.succeed(
+            "docker rmi ${examples.bashNoTagStreamLayered.imageName}:${examples.bashNoTagStreamLayered.imageTag}"
+        )
+
+        docker.succeed(
+            "docker load --input='${examples.nixLayered}'"
+        )
+        docker.succeed("docker images --format '{{.Tag}}' | grep -F '${examples.nixLayered.imageTag}'")
+        docker.succeed("docker rmi ${examples.nixLayered.imageName}")
+
+
     with subtest(
         "Check if the nix store is correctly initialized by listing "
         "dependencies of the installed Nix binary"

pkgs/build-support/docker/default.nix

@@ -442,6 +442,7 @@ rec {
     in
       runCommand "${name}.tar.gz" {
         inherit (stream) imageName;
+        passthru = { inherit (stream) imageTag; };
         buildInputs = [ pigz ];
       } "${stream} | pigz -nT > $out";
 
@@ -517,6 +518,11 @@ rec {
         layerClosure = writeReferencesToFile layer;
         passthru.buildArgs = args;
         passthru.layer = layer;
+        passthru.imageTag =
+          if tag != null
+            then lib.toLower tag
+            else
+              lib.head (lib.strings.splitString "-" (baseNameOf result.outPath));
         # Docker can't be made to run darwin binaries
         meta.badPlatforms = lib.platforms.darwin;
       } ''
@@ -737,6 +743,11 @@ rec {
       conf = runCommand "${name}-conf.json" {
         inherit maxLayers created;
         imageName = lib.toLower name;
+        passthru.imageTag =
+          if tag != null
+            then tag
+            else
+              lib.head (lib.strings.splitString "-" (baseNameOf conf.outPath));
         paths = referencesByPopularity overallClosure;
         buildInputs = [ jq ];
       } ''
@@ -792,6 +803,7 @@ rec {
       '';
       result = runCommand "stream-${name}" {
         inherit (conf) imageName;
+        passthru = { inherit (conf) imageTag; };
         buildInputs = [ makeWrapper ];
       } ''
         makeWrapper ${streamScript} $out --add-flags ${conf}

pkgs/build-support/docker/examples.nix

@@ -364,4 +364,22 @@ rec {
     created = "now";
   };
 
+  # buildImage without explicit tag
+  bashNoTag = pkgs.dockerTools.buildImage {
+    name = "bash-no-tag";
+    contents = pkgs.bashInteractive;
+  };
+
+  # buildLayeredImage without explicit tag
+  bashNoTagLayered = pkgs.dockerTools.buildLayeredImage {
+    name = "bash-no-tag-layered";
+    contents = pkgs.bashInteractive;
+  };
+
+  # buildImage without explicit tag
+  bashNoTagStreamLayered = pkgs.dockerTools.streamLayeredImage {
+    name = "bash-no-tag-stream-layered";
+    contents = pkgs.bashInteractive;
+  };
+
 }