Add missing runtime dependencies to Docker image (#1508)
Add missing runtime dependencies to Docker image (#1508)

* Add missing runtime dependencies to Docker image

This adds dependencies highlighted by `thumbor-doctor` to the Docker
image which allows Thumbor to work better.

* Remove ffmpeg as a dependency

It is only needed for converting GIFs to video formats and pulls a lot
of dependencies with it.

* Remove libexiv2 as a runtime dependency

It was left in by mistake after trying to get py3exiv2 working.
thumbor/thumbor,thumbor/thumbor,thumbor/thumbor,thumbor/thumbor