PostGIS – How to Set a Global Setting in PostgreSQL 9.5/PostGIS 2.2

I am trying to set a global setting/GUC variable postgis.gdal_enabled_drivers from the PostGIS extension for a while. I am a non-admin user but can ask admin to change settings if needed. But I tried several different methods following the instructions but none worked.

1. I tried the old fashion way to change environmental variables. I had the following added to /etc/environment:

   POSTGIS_ENABLE_OUTDB_RASTERS=1
   POSTGIS_GDAL_ENABLED_DRIVERS=ENABLE_ALL
   

so that echo $POSTGIS_GDAL_ENABLED_DRIVERS returns ENABLE_ALL

Now, in my non-admin PostreSQL account,

SELECT short_name, long_name FROM ST_GdalDrivers();

returns 0 rows, meaning no GDAL drivers are enabled.

2. I also tried to have postgres to:

   ALTER SYSTEM SET postgis.gdal_enabled_drivers TO 'ENABLE_ALL';
   

but I got an error:

ERROR:  unrecognized configuration parameter "postgis.gdal_enabled_drivers"

3. I tried to:

   ALTER DATABASE my_db SET postgis.gdal_enabled_drivers TO 'ENABLE_ALL';
   

but I got:

ERROR:  permission denied to set parameter "postgis.gdal_enabled_drivers"

In sum, I can ask admin to change some settings once as in methods 1 and 2, but not every time e.g. when I do a full database restore (method 3).

My question is:

How can I make the above methods work?

FYI, my PostgreSQL version is 9.5.1, OS is Lubuntu 16.04 beta

    postgis_version            
---------------------------------------
 2.2 USE_GEOS=1 USE_PROJ=1 USE_STATS=1

In method 3, I can use postgres to ALTER DATABASE but I can't ask amin to do that every time I restore db, where I do something like:

dropdb my_db && createdb -T template my_db $$ psql < my_pg_dump_file