Update Docker build/image
Update Docker build/image

- Update build container to golang:1.12.4-alpine
- Update dex image to alpine:3.9
- Run dex as non-root user

Signed-off-by: Stephen Augustus <f96984fef9637085f0559f1484dfd4e99545cd6e@vmware.com>

wolfeidau/dex,coreos/dex,wolfeidau/dex,ericchiang/dex,ericchiang/dex,ericchiang/dex,wolfeidau/dex,coreos/dex,coreos/dex