fix(server): fix version parsing

[sbruens]

#1535 changed how we build the Shadowbox server image. Nothing is setting the SB_VERSION environment variable that the webpack config parses to determine the server's version.

task shadowbox:docker:start IMAGE_VERSION="1.1.1"

This now sets the correct version again:

curl --insecure https://[::]:8081/TestApiPrefix/server | jq
{
   ...
  "version": "1.1.1",
   ...
}