Dockerfile: bump Go to 1.16, set ANDROID_SDK_ROOT for dockershell

Dockerfile

@@ -32,12 +32,13 @@ RUN echo y | $ANDROID_HOME/tools/bin/sdkmanager 'build-tools;28.0.3'
 
 # Get Go stable release
 WORKDIR $HOME
-ARG GO_VERSION=1.15.5
+ARG GO_VERSION=1.16.5
 RUN curl -O https://storage.googleapis.com/golang/go${GO_VERSION}.linux-amd64.tar.gz
-RUN echo "9a58494e8da722c3aef248c9227b0e9c528c7318309827780f16220998180a0d  go${GO_VERSION}.linux-amd64.tar.gz" | sha256sum -c
+RUN echo "b12c23023b68de22f74c0524f10b753e7b08b1504cb7e417eccebdd3fae49061  go${GO_VERSION}.linux-amd64.tar.gz" | sha256sum -c
 RUN tar -xzf go${GO_VERSION}.linux-amd64.tar.gz && mv go goroot
 ENV GOROOT $HOME/goroot
 ENV PATH $PATH:$GOROOT/bin:$HOME/bin:$ANDROID_HOME/platform-tools
+ENV ANDROID_SDK_ROOT /build/android-sdk
 
 RUN mkdir -p $HOME/tailscale-android
 WORKDIR $HOME/tailscale-android