build: run vsce package during testing

This will detect npm dependency issues that are detected during
packaging.

Change-Id: I7e3bbeee407b764a18e80a21999385472812cf08
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/279012
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
diff --git a/build/Dockerfile b/build/Dockerfile
index 62a5b98..2d70051 100644
--- a/build/Dockerfile
+++ b/build/Dockerfile
@@ -27,6 +27,7 @@
 ENV DEBIAN_FRONTEND noninteractive
 
 RUN apt-get update && apt-get install -y libnss3 libgtk-3-dev libxss1 libasound2 xvfb libsecret-1-0
+RUN npm install -g vsce
 
 WORKDIR /workspace
 ENTRYPOINT ["build/all.bash"]
diff --git a/build/all.bash b/build/all.bash
index f91df52..afd0706 100755
--- a/build/all.bash
+++ b/build/all.bash
@@ -52,6 +52,9 @@
 
   echo "**** Run settings generator ****"
   go run tools/generate.go -w=false
+
+  echo "**** Check if vsce works ****"
+  vsce package
 }
 
 run_test_in_docker() {