Switch cross builds to use BUILD_IN_DOCKER

minikube-linux-amd64 and minikube-darwin-amd64 cannot be cross built
natively on different platforms and must be built in the container.

docs/contributors/releasing_minikube.md

@@ -40,7 +40,7 @@ Send a PR for the Makefile change and wait until it is merged.  Once the commit
 Run this command:
 
 ```shell
-make cross checksum
+BUILD_IN_DOCKER=y make cross checksum
 ```
 
 ## Add the version to the releases.json file

hack/jenkins/minikube_cross_build_and_upload.sh

@@ -38,7 +38,7 @@ fi
 export GOPATH=~/go
 
 # Build all platforms (Windows, Linux, OSX)
-make cross
+BUILD_IN_DOCKER=y make cross
 
 # Build the e2e test target for Darwin and Linux. We don't run tests on Windows yet.
 # We build these on Linux, but run the tests on different platforms.

hack/jenkins/release_build_and_upload.sh

@@ -38,7 +38,7 @@ cat Makefile | grep "VERSION_MINOR ?=" | grep $VERSION_MINOR
 cat Makefile | grep "VERSION_BUILD ?=" | grep $VERSION_BUILD
 
 # Build and upload
-make cross checksum
+BUILD_IN_DOCKER=y make cross checksum
 
 gsutil cp out/minikube-linux-amd64 gs://$BUCKET/releases/$TAGNAME/
 gsutil cp out/minikube-linux-amd64.sha256 gs://$BUCKET/releases/$TAGNAME/