dockerTools: refactor, rename internal variable

> has to fit its domain, which is the OCI spec, which uses
> `architecture`. The `defaultArch` and `GOARCH` names are irrelevant.

pkgs/build-support/docker/default.nix

@@ -74,7 +74,7 @@ let
   # Reference: https://github.com/opencontainers/image-spec/blob/master/config.md#properties
   # For the mapping from Nixpkgs system parameters to GOARCH, we can reuse the
   # mapping from the go package.
-  defaultArch = go.GOARCH;
+  defaultArchitecture = go.GOARCH;
 
 in
 rec {
@@ -102,7 +102,7 @@ rec {
     , sha256
     , os ? "linux"
     , # Image architecture, defaults to the architecture of the `hostPlatform` when unset
-      arch ? defaultArch
+      arch ? defaultArchitecture
       # This is used to set name to the pulled image
     , finalImageName ? imageName
       # This used to set a tag to the pulled image
@@ -515,7 +515,7 @@ rec {
     , # Docker config; e.g. what command to run on the container.
       config ? null
     , # Image architecture, defaults to the architecture of the `hostPlatform` when unset
-      architecture ? defaultArch
+      architecture ? defaultArchitecture
     , # Optional bash script to run on the files prior to fixturizing the layer.
       extraCommands ? ""
     , uid ? 0
@@ -840,7 +840,7 @@ rec {
     , # Docker config; e.g. what command to run on the container.
       config ? { }
     , # Image architecture, defaults to the architecture of the `hostPlatform` when unset
-      architecture ? defaultArch
+      architecture ? defaultArchitecture
     , # Time of creation of the image. Passing "now" will make the
       # created date be the time of building.
       created ? "1970-01-01T00:00:01Z"