Migrate text from imagej.github.io

[joshmoore]

see: https://github.com/imagej/imagej.github.io/blob/9a225cd551e49a0f3bdd0267000c07b3e15247e7/_pages/learn/docker.md

Changes made from that text:

• re-indented blocks
• replaced {% include %} with a :warning:
• changed /learn/ relative URLs to absolutes
• added a badge (that at least in the PR isn't working)
• modified all heading levels
• replaced "mailing list" with "image.sc"

Closes #17.

[ctrueden]

@joshmoore Thanks for your efforts on this. I did a cleanup pass through the README in the aftermath.

I don't have time right now to test on my systems. I am interested in trying it out when I have time, perhaps in mid-May.

But in the meantime, I was wondering: do you ever use Docker with graphical applications on your systems? If so, how? Have you seen x11docker, and if so, have you tried it? The GUI-related instructions in the README here are very complex, and I am guessing quite outdated—for example we still talk about boot2docker for macOS but Docker Desktop is the thing now, so... maybe that whole section becomes much simpler now? Do you know?

Edit: Also, I see that the directories in this repository no longer match the tags you deployed. Would it make sense to have e.g. a fiji-openjdk-11 directory in this repository? And/or retire the Dockerfiles for now-obsolete versions of Java 6 and 7? How are you building the openjdk-11 images?