Add protoc release script for Linux build.

kokoro/release/protoc/linux/build.sh

+#!/bin/bash
+
+set -x
+
+# Change to repo root.
+cd $(dirname $0)/../../../..
+
+# Use docker image to build linux artifacts.
+DOCKER_IMAGE_NAME=protobuf/protoc_$(sha1sum protoc-artifacts/Dockerfile | cut -f1 -d " ")
+docker pull $DOCKER_IMAGE_NAME
+
+docker run -v $(pwd):/var/local/protobuf --rm $DOCKER_IMAGE_NAME \
+    bash -l /var/local/protobuf/protoc-artifacts/build-protoc.sh \
+    linux x86_64 protoc || {
+  echo "Failed to build protoc for linux + x86_64."
+  exit 1
+}
+
+docker run -v $(pwd):/var/local/protobuf --rm $DOCKER_IMAGE_NAME \
+    bash -l /var/local/protobuf/protoc-artifacts/build-protoc.sh \
+    linux x86_32 protoc || {
+  echo "Failed to build protoc for linux + x86_32."
+  exit 1
+}
+
+# Cross-build for some architectures.
+sudo apt install g++-aarch64-linux-gnu
+# TODO(xiaofeng): It currently fails with "machine `aarch64' not recognized"
+# error.
+# protoc-artifacts/build-protoc.sh linux aarch_64 protoc

kokoro/release/protoc/linux/release.cfg

+build_file: "protobuf/kokoro/release/protoc/linux/build.sh"
+
+action {
+  define_artifacts {
+    regex: "**/protoc.exe"
+  }
+}

protoc-artifacts/build-protoc.sh

@@ -38,7 +38,7 @@ case $MAKE_TARGET in
   protoc)
     ;;
   *)
-    echo "Target ""$TARGET"" invalid."
+    echo "Target ""$MAKE_TARGET"" invalid."
     exit 1
 esac
 
@@ -165,7 +165,7 @@ checkDependencies ()
 }
 ############################################################################
 
-echo "Building protoc, OS=$OS ARCH=$ARCH TARGET=$TARGET"
+echo "Building protoc, OS=$OS ARCH=$ARCH TARGET=$MAKE_TARGET"
 
 CONFIGURE_ARGS="--disable-shared"