Build done 
The most disk space used was :
Aug 22 13:51:02 344G /srv
Aug 22 13:55:09 353G /srv
Aug 22 14:00:05 303G /srv
Corresponding entries in log (Docker is 2 hours behind my local clock) :
2021-08-22 11:54:52 - common.py - INFO : using prebuilt recovery.img from BOOTABLE_IMAGES…
2021-08-22 11:54:52 - common.py - INFO : Running: “brotli --quality=6 --output=/srv/src/Q/out/soong/.temp/tmpw7Htqs/system.new.dat.br /srv/src/Q/out/soong/.temp/tmpw7Htqs/system.new.dat”
2021-08-22 11:57:07 - common.py - INFO :
2021-08-22 11:57:09 - common.py - INFO : using prebuilt boot.img from BOOTABLE_IMAGES…
So, the most usage was at the end of the build process, when the system image was assembled.
That make sense
NB 1 : my CCACHE setting was the default 50G, after build the size of ccache directory is 11G …
NB 2 : the du command ran every 5 minutes, so real max size can be a little higher …