I wouldn't call it a lot. What you mention about php, for example, is on the first dockerhub page of PHP's official image.
Also, Docker (and most cloud native technologies) exist to enable devops such that development teams are enabled to deliver their applications. If you're a Ruby developer, you sure need to learn the quirks of official Ruby ages (and perhaps how to build one from scratch) but definitely not all language based images.
Finally, there are many other base images depending on how to deliver applications: for rails, for symfony, for roadrunner. One does not preemptively learn all.
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I wouldn't call it a lot. What you mention about php, for example, is on the first dockerhub page of PHP's official image.
Also, Docker (and most cloud native technologies) exist to enable devops such that development teams are enabled to deliver their applications. If you're a Ruby developer, you sure need to learn the quirks of official Ruby ages (and perhaps how to build one from scratch) but definitely not all language based images.
Finally, there are many other base images depending on how to deliver applications: for rails, for symfony, for roadrunner. One does not preemptively learn all.
Those are all fair points.