Update readme of alpine based images to warn about DNS issue with very long hostnames

[ribalba]

Images that have alpine:

• idle
• puppeteer chrome/firefox
• stress
• network-test
• Idle and stress don't really use any networking, maybe skip those?

Rationale for this issue: https://purplecarrot.co.uk/post/2021-09-04-does_alpine-resolve_dns_properly/

new issue due to @djesic not wanting issues in his name: #36