playground: put GOCACHE in temp dir

Fixes golang/go#30473

Change-Id: Iad5b60b98c12add932fa3be9174fe7ca4dcbb6b2
Reviewed-on: https://go-review.googlesource.com/c/164519
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Dockerfile

diff --git a/Dockerfile b/Dockerfile
index c196207..c6390fc 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -222,10 +222,6 @@
 COPY --from=builder /usr/local/go /usr/local/go
 COPY --from=builder /tmp/sel_ldr_x86_64 /usr/local/bin
 
-# For implicit GOCACHE (issues 29243 and 29251), set HOME:
-RUN mkdir -p /home/gopher
-ENV HOME /home/gopher
-
 ENV GOPATH /go
 ENV PATH /usr/local/go/bin:$GOPATH/bin:$PATH
 

sandbox.go

diff --git a/sandbox.go b/sandbox.go
index 8e21a96..7493213 100644
--- a/sandbox.go
+++ b/sandbox.go
@@ -323,8 +323,9 @@
 	}
 
 	exe := filepath.Join(tmpDir, "a.out")
+	goCache := filepath.Join(tmpDir, "gocache")
 	cmd := exec.Command("go", "build", "-o", exe, in)
-	cmd.Env = []string{"GOOS=nacl", "GOARCH=amd64p32", "GOPATH=" + os.Getenv("GOPATH"), "HOME=" + os.Getenv("HOME")}
+	cmd.Env = []string{"GOOS=nacl", "GOARCH=amd64p32", "GOPATH=" + os.Getenv("GOPATH"), "GOCACHE=" + goCache}
 	if out, err := cmd.CombinedOutput(); err != nil {
 		if _, ok := err.(*exec.ExitError); ok {
 			// Return compile errors to the user.