Fix broken Dockerfile - #640

Hi! Thanks for providing the Dockerfile here. I just ran vsce for the first time to publish my first Visual Studio Code extension. I needed to fix a couple of things in the Dockerfile though to get it working with my environment:

Building the image using the instructions in the README

$ docker build -t vsce .

fails with

Step 3/10 : COPY package.json package-lock.json .
When using COPY with more than one source file, the destination must be a directory and end with a /

so I just added the /.

Then there were issues with the ENTRYPOINT when running the container:

$ docker run -it vsce --version
docker: Error response from daemon: OCI runtime create failed: container_linux.go:380: starting container process caused: exec: "/opt/vsce/out/vsce": stat /opt/vsce/out/vsce: no such file or directory: unknown.
ERRO[0001] error waiting for container: context canceled 

I just fixed the path /opt/vsce/vsce, but I haven't dug any deeper to check if the path is actually supposed to be /opt/vsce/out/vsce and there's a better way of fixing this issue.

I'm running Debian buster with

$ docker --version
Docker version 20.10.7, build f0df350