I found a bug at the v0.1.9-1 version of s6opts. A new release (v0.1.9-2) can be found on the testing repo, this release fix it.
Append your ~/service/service.conf with this :
# Verbosity for s6 command
# 0 silences warnings.
# 1 warning and error messages will be printed to stderr.
# 2 adds a bit more information about what s6-rc-compile is doing.
# 3 or more is heavy debug output.
VERBOSE=1
Obviously the comment part is not mandatory :).
Without this variable, it's impossible to build database with classic user privilegies.
NOTE : Thank you at the dev to place the new version of s6opts on testing repo first :D