update Dockerfile (#1152)
update Dockerfile (#1152)

- use ruby:2.3.3-alpine image
- add EXPOSE
- install deps first to leverage layer cache for dev
Cloud-Temple/errbit,errbit/errbit,stevecrozz/errbit,openSUSE/errbit,iFixit/errbit,openSUSE/errbit,ninech/errbit,cognoa/errbit,mikk150/errbit,disvelop/errbit,jordoh/errbit,cognoa/errbit,mak-it/errbit,threatsim/errbit,rud/errbit,iFixit/errbit,jordoh/errbit,stevecrozz/errbit,wombatsecurity/errbit,openSUSE/errbit,stevecrozz/errbit,wombatsecurity/errbit,errbit/errbit,threatsim/errbit,mak-it/errbit,rud/errbit,mikk150/errbit,diowa/errbit,jordoh/errbit,mikk150/errbit,dwilkie/errbit,disvelop/errbit,iFixit/errbit,micred/errbit,disvelop/errbit,threatsim/errbit,mak-it/errbit,wombatsecurity/errbit,errbit/errbit,diowa/errbit,cognoa/errbit,bonanza-market/errbit,rud/errbit,dwilkie/errbit,ninech/errbit,diowa/errbit,Cloud-Temple/errbit,wombatsecurity/errbit,Cloud-Temple/errbit,micred/errbit,bonanza-market/errbit,ninech/errbit,micred/errbit,bonanza-market/errbit,threatsim/errbit,dwilkie/errbit