Add version tag(s)

It would be nice to use this in local development scripts. In order to avoid `docker pull` every time a container is started, it would be super helpful to have version tags. Then you can just do

```
docker run --restart=always -d -p 3000:3000 --name cors-container \
  imjacobclark/cors-container:v0.1.2
```

which will automatically pull the image if missing and use from cache if available.