Document Ferrum's Docker container mode

[ablzh]

Summary

• replace the explicit no-sandbox example with Ferrum's dockerize: true
  option
• clarify that container mode is needed only when Chrome or Chromium cannot
  use its sandbox
• recommend keeping the browser sandbox enabled when the container supports it

Why

Ferrum 0.17.2 introduced the dockerize: true option in
rubycdp/ferrum#561.
The same change updated Ferrum's Docker documentation to use dockerize: true
instead of configuring no-sandbox manually.

Cuprite's browser inherits from Ferrum::Browser and passes its driver options
to Ferrum, so the option can be used directly in the Cuprite driver
configuration. The current Cuprite README still shows the older manual option
and implies that disabling the sandbox is required for every Docker setup.

This change aligns the Cuprite documentation with Ferrum's current container
API while making it clear that the browser sandbox should remain enabled when
the container supports it.

References

• Ferrum 0.17.2 changelog
• Ferrum dockerize implementation
• Ferrum PR introducing container mode
• Cuprite browser inheritance

Testing

Documentation-only change. No automated tests were run.

[route]

Thanks!