Base image size is unnecessarily large

[pelithne]

The base container for the azure-vote-app is 922 MB in size. This can be improved significantly, by using an Alpine based image instead of Ubuntu. This is good for at least two reasons:
1: Download/upload times decreases which is very good in e.g. a workshop where several participants share the same WIFI (talking from experience here...)

2: Best practice with containers is to keep them as small as possible

I can make a PR with a smaller image referenced in the Dockerfile, if that would be appreciated