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.