add shebang to test script; install bash in container

Jenkinsfile

@@ -6,7 +6,7 @@ pipeline {
         checkout scm
         script {
           docker.image("golang:1.14-alpine").inside {
-            sh "apk add --no-cache protobuf-dev"
+            sh "apk add --no-cache protobuf-dev bash"
             sh "go get -u github.com/golang/protobuf/protoc-gen-go"
             sh "./test.sh"
           }

test.sh

@@ -1,3 +1,4 @@
+#!/bin/bash
 set -Eeuxo pipefail
 
 # ./tdb/stringy