Add documentation for xfce-test docker environment
## Submitted by Kevin Bowen `@kevinbowen` Assigned to **Kevin Bowen `@kevinbowen`** **[Link to original bug (#16249)](https://bugzilla.xfce.org/show_bug.cgi?id=16249)** ## Description Add information under development tools https://docs.xfce.org/contribute/dev/start regarding use of xfce-test docker environment used for testing Xfce builds https://github.com/schuellerf/xfce-test Could use some beginner info on getting it downloaded, installed, and started with docker
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