Add a simple way to run grpc-java test inside emulated ARM64 docker container

.gitignore

@@ -35,3 +35,6 @@ bin
 # Emacs
 *~
 \#*\#
+
+# ARM tests
+qemu-arm-static

buildscripts/grpc-java-linux-arm64-tests/Dockerfile

@@ -0,0 +1,10 @@
+# This docker image uses magic to seamlessly run its contents using qemu ARM emulator on x86 host.
+# See https://ownyourbits.com/2018/06/27/running-and-building-arm-docker-containers-in-x86/
+
+# We want to test on arm64v8
+FROM arm64v8/debian:buster
+
+# Make the host's qemu-arm-static emulator available from inside the docker container.
+COPY qemu-arm-static /usr/bin
+
+RUN apt-get update && apt-get install -y --no-install-recommends openjdk-11-jdk-headless && apt-get clean

buildscripts/run_arm64_tests_in_docker.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+set -ex
+
+readonly grpc_java_dir="$(dirname "$(readlink -f "$0")")/.."
+
+# Build magic docker image that can run on x86 host, but looks like an ARM machine
+# from the inside (qemu-user-static is used for emulation)
+# Run "sudo apt install qemu-user-static binfmt-support" install the emulator
+# on the host machine.
+# Also see https://ownyourbits.com/2018/06/27/running-and-building-arm-docker-containers-in-x86/
+cp /usr/bin/qemu-arm-static "${grpc_java_dir}/buildscripts/grpc-java-linux-arm64-tests"
+docker build -t grpc-java-linux-arm64-tests "${grpc_java_dir}/buildscripts/grpc-java-linux-arm64-tests"
+
+if [[ -t 0 ]]; then
+  DOCKER_ARGS="-it"
+else
+  # The input device on kokoro is not a TTY, so -it does not work.
+  DOCKER_ARGS=
+fi
+
+# - run docker container under current user's UID to avoid polluting the workspace
+# - set the user.home property to avoid creating a "?" directory under grpc-java
+exec docker run $DOCKER_ARGS --rm=true -v "${grpc_java_dir}":/grpc-java -w /grpc-java \
+  --user "$(id -u):$(id -g)" -e "JAVA_OPTS=-Duser.home=/grpc-java/.current-user-home" \
+  grpc-java-linux-arm64-tests \
+  bash -c "./gradlew build -PskipAndroid=true -PskipCodegen=true"