feat: add env expansion, sandbox env fetching, and configurable

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.

internal/recipe/executor.go

@@ -68,7 +68,7 @@ func Execute(
 			if bctx.EnvVars == nil {
 				bctx.EnvVars = make(map[string]string)
 			}
-			bctx.EnvVars[st.Key] = st.Value
+			bctx.EnvVars[st.Key] = expandEnv(st.Value, bctx.EnvVars)
 			entries = append(entries, BuildLogEntry{Step: step, Phase: phase, Cmd: st.Raw, Ok: true})
 
 		case KindWORKDIR: