Fixing the wrong Git Commit hash in docker version

`DOCKER_GITCOMMIT` needs to match the tagged commit used to build the
binary. The current commit refers to 1.12.1 and wasn't update each
time we updated the package. Using a variable near the version and
adding a comment so we don't forget to update next time.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

pkgs/applications/virtualization/docker/default.nix

@@ -12,6 +12,7 @@ with lib;
 stdenv.mkDerivation rec {
   name = "docker-${version}";
   version = "1.13.0";
+  rev = "49bf474"; # should match the version commit
 
   src = fetchFromGitHub {
     owner = "docker";
@@ -79,7 +80,7 @@ stdenv.mkDerivation rec {
   buildPhase = ''
     patchShebangs .
     export AUTO_GOPATH=1
-    export DOCKER_GITCOMMIT="23cf638"
+    export DOCKER_GITCOMMIT="${rev}"
     ./hack/make.sh dynbinary
   '';