diff --git a/Dockerfile b/Dockerfile
index 3ea5419..7ddad2a 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -17,6 +17,7 @@
 ENV GOPATH /go
 ENV PATH /usr/local/go/bin:$GOPATH/bin:$PATH
 ENV GOROOT_BOOTSTRAP /usr/local/gobootstrap
+ENV GO_BOOTSTRAP_VERSION go1.12.6
 ARG GO_VERSION=go1.12.6
 ENV GO_VERSION ${GO_VERSION}
 
@@ -25,17 +26,19 @@
 # Fake file system
 COPY fake_fs.lst /usr/local/playground/
 
-# Get the Go binary.
-RUN curl -sSL https://dl.google.com/go/$GO_VERSION.linux-amd64.tar.gz -o /tmp/go.tar.gz
-RUN curl -sSL https://dl.google.com/go/$GO_VERSION.linux-amd64.tar.gz.sha256 -o /tmp/go.tar.gz.sha256
+# Get a bootstrap version of Go for building from source.
+RUN curl -sSL https://dl.google.com/go/$GO_BOOTSTRAP_VERSION.linux-amd64.tar.gz -o /tmp/go.tar.gz
+RUN curl -sSL https://dl.google.com/go/$GO_BOOTSTRAP_VERSION.linux-amd64.tar.gz.sha256 -o /tmp/go.tar.gz.sha256
 RUN echo "$(cat /tmp/go.tar.gz.sha256) /tmp/go.tar.gz" | sha256sum -c -
-RUN tar -C /usr/local/ -vxzf /tmp/go.tar.gz
-# Make a copy for GOROOT_BOOTSTRAP, because we rebuild the toolchain and make.bash removes bin/go as its first step.
-RUN cp -R /usr/local/go $GOROOT_BOOTSTRAP
+RUN mkdir -p $GOROOT_BOOTSTRAP
+RUN tar --strip=1 -C $GOROOT_BOOTSTRAP -vxzf /tmp/go.tar.gz
+
+# Fetch Go source for tag $GO_VERSION.
+RUN git clone --depth=1 --branch=$GO_VERSION https://go.googlesource.com/go /usr/local/go
 # Apply the fake time and fake filesystem patches.
 RUN patch /usr/local/go/src/runtime/rt0_nacl_amd64p32.s /usr/local/playground/enable-fake-time.patch
-RUN cd /usr/local/go && go run misc/nacl/mkzip.go -p syscall /usr/local/playground/fake_fs.lst src/syscall/fstest_nacl.go
-# Re-build the Go toolchain.
+RUN cd /usr/local/go && $GOROOT_BOOTSTRAP/bin/go run misc/nacl/mkzip.go -p syscall /usr/local/playground/fake_fs.lst src/syscall/fstest_nacl.go
+# Build the Go toolchain.
 RUN cd /usr/local/go/src && GOOS=nacl GOARCH=amd64p32 ./make.bash --no-clean
 
 RUN mkdir /gocache
diff --git a/cloudbuild_trigger.json b/cloudbuild_trigger.json
index 2fa8304..d37d803 100644
--- a/cloudbuild_trigger.json
+++ b/cloudbuild_trigger.json
@@ -20,15 +20,29 @@
         ]
       },
       {
+        "name": "golang",
+        "entrypoint": "bash",
+        "args": [
+          "-c",
+          "go get golang.org/x/build/maintner/maintq && cp /go/bin/maintq /workspace/maintq"
+        ]
+      },
+      {
+        "name": "debian:stretch",
+        "entrypoint": "bash",
+        "args": [
+          "-c",
+          "apt-get update && apt-get install -y ca-certificates && ./maintq list-releases | head -n 1 | sed -re 's/^.*tag_name:\"(go[0-9](\\.[0-9]+)+)\".*$/\\1/g' > /workspace/latestgo"
+        ]
+      },
+      {
         "dir": "playground",
         "name": "gcr.io/cloud-builders/docker",
+        "entrypoint": "bash",
         "args": [
-          "build",
-          "-t",
-          "gcr.io/$PROJECT_ID/playground",
-          "."
-        ],
-        "timeout": "1800s"
+          "-c",
+          "docker build --build-arg GO_VERSION=`cat /workspace/latestgo` -t gcr.io/$PROJECT_ID/playground ."
+        ]
       },
       {
         "dir": "playground",
@@ -39,10 +53,10 @@
           "app.yaml",
           "--project=$PROJECT_ID",
           "--image-url=gcr.io/$PROJECT_ID/playground:latest"
-        ],
-        "timeout": "1200s"
+        ]
       }
     ],
+    "timeout": "1800s",
     "images": [
       "gcr.io/$PROJECT_ID/playground"
     ]