Document which ST_GDALDrivers are available on debian vs alpine based images

[zmbm-dev]

Small documentation request. I was trying to export part of a raster using ST_AsPNG and it took me a while to figure out that the PNG GDAL driver isn't available on the alpine image.

SET postgis.gdal_enabled_drivers = 'ENABLE_ALL';
SELECT short_name, long_name, can_write
FROM st_gdaldrivers()
ORDER BY short_name;

showed no PNG driver but google didn't give any hint as to why.

Looking at the gdal documentation, it's built-in by default so I was confused for a bit. Eventually I tried the debian based image which includes the PNG driver, but I thought it'd be nice to have this documented somewhere.

[AHB102]

@evanderh Can I document this ?

[zmbm-dev]

@AHB102 Go for it!