Enable go module support for go environment (#1152)

environments/go/builder/build.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 set -eux
 
@@ -6,6 +6,9 @@ srcDir=${GOPATH}/src/$(basename ${SRC_PKG})
 
 trap "rm -rf ${srcDir}" EXIT
 
+# http://ask.xmodulo.com/compare-two-version-numbers.html
+version_ge() { test "$(echo "$@" | tr " " "\n" | sort -rV | head -n 1)" == "$1"; }
+
 if [ -d ${SRC_PKG} ]
 then
     echo "Building in directory ${srcDir}"
@@ -18,4 +21,17 @@ then
 fi
 
 cd ${srcDir}
+
+if [ -f "go.mod" ]; then
+    if [ ! -z ${GOLANG_VERSION} ] && version_ge ${GOLANG_VERSION} "1.12"; then
+        go mod download
+    else
+        echo "Please update fission/go-builder and fission/go-env image to the latest version to support go module"
+        exit 1
+    fi
+else
+    # still need to do this; otherwise, go will complain "cannot find main module".
+    go mod init
+fi
+
 go build -buildmode=plugin -i -o ${DEPLOY_PKG} .