build-support/docker: set default image arch to host arch

The architecture of an image should default to the architecture for
which that image is being composed or pulled. buildPackages.go.GOARCH is
an easy way to compute that architecture with the correct terminology.

pkgs/build-support/docker/default.nix

@@ -1,4 +1,5 @@
 {
+  buildPackages,
   cacert,
   callPackage,
   closureInfo,
@@ -47,7 +48,7 @@ rec {
     , imageDigest
     , sha256
     , os ? "linux"
-    , arch ? "amd64"
+    , arch ? buildPackages.go.GOARCH
 
       # This is used to set name to the pulled image
     , finalImageName ? imageName
@@ -540,7 +541,7 @@ rec {
       configJson = let
           pure = writeText "${baseName}-config.json" (builtins.toJSON {
             inherit created config;
-            architecture = "amd64";
+            architecture = buildPackages.go.GOARCH;
             os = "linux";
           });
           impure = runCommand "${baseName}-standard-dynamic-date.json"
@@ -658,7 +659,7 @@ rec {
       baseJson = let
           pure = writeText "${baseName}-config.json" (builtins.toJSON {
             inherit created config;
-            architecture = "amd64";
+            architecture = buildPackages.go.GOARCH;
             os = "linux";
           });
           impure = runCommand "${baseName}-config.json"