[PostGIS] #5950: Document the use of POSTGIS_REGRESS_DB_OWNER

[PostGIS]

#5950: Document the use of POSTGIS_REGRESS_DB_OWNER
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 Reporter:  robe           |      Owner:  robe
     Type:  defect         |     Status:  new
 Priority:  medium         |  Milestone:  PostGIS 3.6.0
Component:  documentation  |    Version:  3.5.x
 Keywords:                 |
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 The variable POSTGIS_REGRESS_DB_OWNER was introduced to allow sandboxed
 accounts to be able to do regression testing.

 Without setting this, sandboxed accounts will run into issues as noted in

 #5212

 This should be documented in docs and development wiki
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/5950>
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