Make nokogiri use the bundled version of libxml2 instead of the system
version. In the past installing nokogiri was slow because it had to
compile the bundled version of libxml2, which is partly why we switched
to the system library. Now it's faster because the bundled version comes
pre-compiled with the nokogiri gem.
https://nokogiri.org/#native-gems-faster-more-reliable-installation
Reverts 440bbbb28.
Fix the ca-certificates package not being installed inside the base
Docker image. This caused uploads from HTTPS sites to fail because TLS
certificates couldn't be validated.
* Optimize Dockerfile to minimize size of the Docker image.
* Specify exact versions of important dependencies (Ruby, Node, Vips) to
ensure our dependencies are up to date and locked to known versions.
* Install Vips from source because the version that ships with Ubuntu is too old.
* Install FFmpeg from source because otherwise using the Ubuntu package
pulls in tons of video libraries we don't need, bloating the image.