dockerTools: Fix layer redundancy.

When building an image with multiple layers, files
already included in an underlying layer are supposed to
be excluded from the current layer. However, some subtleties
in the way filepaths are compared seem to be blocking this.

Specifically:
* tar generates relative filepaths with directories ending in '/'
* find generates absolute filepaths with no trailing slashes on directories

That is, paths extracted from the underlying tarball look like:
    nix/store/.../foobar/
whereas the layer being generated uses paths like:
    /nix/store/.../foobar

This patch modifies the output of "tar -t" to match the latter format.

pkgs/build-support/docker/default.nix

@@ -1,5 +1,5 @@
 { stdenv, lib, callPackage, runCommand, writeReferencesToFile, writeText, vmTools, writeScript
-, docker, shadow, utillinux, coreutils, jshon, e2fsprogs, go, pigz }:
+, docker, shadow, utillinux, coreutils, jshon, e2fsprogs, go, pigz, findutils }:
 
 # WARNING: this API is unstable and may be subject to backwards-incompatible changes in the future.
 
@@ -249,7 +249,7 @@ EOF
                then mkPureLayer { inherit baseJson contents extraCommands; }
                else mkRootLayer { inherit baseJson fromImage fromImageName fromImageTag contents runAsRoot diskSize extraCommands; });
       result = runCommand "${baseName}.tar.gz" {
-        buildInputs = [ jshon pigz ];
+        buildInputs = [ jshon pigz coreutils findutils ];
 
         imageName = name;
         imageTag = tag;
@@ -261,6 +261,18 @@ EOF
           buildArgs = args;
         };
       } ''
+        # Print tar contents:
+        # 1: Interpreted as relative to the root directory
+        # 2: With no trailing slashes on directories
+        # This is useful for ensuring that the output matches the values generated by the "find" command
+        ls_tar() {
+            for f in $(tar -tf $1 | xargs realpath -ms --relative-to=.); do
+                if [ "$f" != "." ]; then
+                    echo "/$f"
+                fi
+            done
+        }
+      
         mkdir image
         touch baseFiles
         if [ -n "$fromImage" ]; then
@@ -276,7 +288,7 @@ EOF
           parentID=$(jshon -e $fromImageName -e $fromImageTag -u < image/repositories)
           
           for l in image/*/layer.tar; do
-            tar -tf $l >> baseFiles
+            ls_tar $l >> baseFiles
           done
         fi
 
@@ -297,8 +309,7 @@ EOF
         fi
 
         echo Adding layer
-        tar -tf temp/layer.tar >> baseFiles
-        sed 's/^\.//' -i baseFiles
+        ls_tar temp/layer.tar >> baseFiles
         comm <(sort -u baseFiles) <(sort -u layerFiles) -1 -3 > newFiles
         tar -rpf temp/layer.tar --mtime=0 --no-recursion --files-from newFiles 2>/dev/null || true