wait_for_template=False raises TemplateBuildingError to poll yourself).
After that it is fast for everyone (public images) or your org (private ones).
Sharp edges
Templates are per(image, cpus, memory_mb, runtime). In the
vm runtime don’t start a second dockerd on dind images — the image’s own
daemon already holds the socket. Omitting the image gives the default
sandbox (Python 3.12, Node 22, common data-science stack, 2 vCPU/1 GiB,
~50–100 ms from a warm pool); it sets PIP_NO_CACHE_DIR=1, so repeated
installs in one sandbox want PIP_NO_CACHE_DIR=0 pip install ….