I think Ubuntu is the most popular distro in the cloud, at least based on cloud provider metrics. Dockerhub shows like 30 million downloads a week for it regularly, which is a lot compared to most images. Debian would be good to learn as that’s what Ubuntu is based on and all the major software with will probably target it. Alpine is good to learn as it’s super slim, tends to be used for containers a lot.

I don't use Linux at work (I wish I did), but I default to Ubuntu Server for at-home Docker needs. I might switch to plain Debian at some point.

I recently finished reading a good docker book. They explained why alpine is so great to use: its like 16 MBs or something. I deployed a Minecraft server with it just for fun. Pretty cool. Shrunk the image a good 15 percent from a debian version I believe. Check it out if you want. Have a good one.