ST_Envelope — Returns the polygon representation of the extent of the raster.
geometry ST_Envelope(raster  rast);
Returns the polygon representation of the extent of the raster in spatial coordinate units defined by srid. It is a float8 minimum bounding box represented as a polygon.
The polygon is defined by the corner points of the bounding box
            ((MINX, MINY),
            (MINX, MAXY),
            (MAXX, MAXY),
            (MAXX, MINY),
            (MINX, MINY))
                    
SELECT rid, ST_AsText(ST_Envelope(rast)) As envgeomwkt
FROM dummy_rast;
 rid |                                         envgeomwkt
-----+--------------------------------------------------------------------
   1 | POLYGON((0 0,20 0,20 60,0 60,0 0))
   2 | POLYGON((3427927 5793243,3427928 5793243,
        3427928 5793244,3427927 5793244, 3427927 5793243))