util-docker: Set the GPU Docker images to build only to x86

These images won't work and make no sense compiling to any platform
other than X86. These are used in SE mode simulations where the host
platform matters.

Change-Id: I47405e930bf511fabcbc93d0b08ee2fb2c556869

util/dockerfiles/docker-bake.hcl

@@ -179,6 +179,7 @@ group "ubuntu-releases" {
 
 target "gcn-gpu" {
   inherits = ["common"]
+  platform = "linux/amd64" # Only build for x86.
   dockerfile = "Dockerfile"
   context = "gcn-gpu"
   tags = ["${IMAGE_URI}/gcn-gpu:${TAG}"]
@@ -186,6 +187,7 @@ target "gcn-gpu" {
 
 target "gpu-fs" {
   inherits = ["common"]
+  platform = "linux/amd64" # Only build for x86.
   dockerfile = "Dockerfile"
   context = "gpu-fs"
   tags = ["${IMAGE_URI}/gpu-fs:${TAG}"]