update Dockerfile to work with current mirage-dev
Merge pull request #320 from lnmx/update-dockerfile

update Dockerfile to work with current mirage-dev
Added tracing support
Merge pull request #321 from talex5/profile

Added tracing support
Merge branch 'master' of github.com:mirage/mirage
Only build mirage-types if Io_page is also installed

Fixes failures such as:
https://travis-ci.org/ocaml/opam-repository/jobs/41615656