healthchecks
Fix ENV instructions to expand $VAR references at set time using the
current env state, preventing self-referencing values like
PATH=/opt/venv/bin:$PATH from producing recursive expansions. Remove
expandEnv from shellPrefix to avoid double expansion.
Fetch sandbox environment variables via `env` before recipe execution
so ENV steps resolve against actual runtime values from the base
template image.
Replace hardcoded healthcheck timing with a Dockerfile-like flag parser
supporting --interval, --timeout, --start-period, and --retries. Add
start-period grace window and bounded retry counting to
waitForHealthcheck.
Add python-interpreter-v0-beta recipe and healthcheck files.
