ORA-01031 on execute immediate ALTER SESSION SET CONTAINER

Hi

Connected with sqlplus SYS as sysdba, I would like to loop on all opened PDBs to execute some task with a plsql loop:

$ORACLE_HOME/bin/sqlplus '/ as sysdba' <<EOF

BEGIN

FOR container IN (SELECT  CON_ID , NAME FROM V\$CONTAINERS WHERE CON_ID > 2 and OPEN_MODE='READ WRITE')

LOOP

EXECUTE IMMEDIATE     'ALTER SESSION SET CONTAINER = ' || container.NAME;

-- execute some task

END LOOP;

END;

/

EOF

But I get an error

ERREUR a la ligne 1 :
ORA-01031: privileges insuffisants

And indeed I just discovered that I'm unable to manually plSQL execute immediate an 'alter session set container' once I'm in the first PDB to swith to next opened PDB, while of course I can change container with SQL "alter session set container" as I'm sys as sysdba…