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/healthcheck.go

@@ -0,0 +1,94 @@
+package recipe
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// HealthcheckConfig holds the parsed configuration for a build healthcheck.
+// A healthcheck is a shell command that is executed repeatedly inside the
+// sandbox until it succeeds or the retry/timeout budget is exhausted.
+//
+// Retries of 0 means unlimited retries (bounded only by the overall deadline)
+type HealthcheckConfig struct {
+	Cmd         string
+	Interval    time.Duration
+	Timeout     time.Duration
+	StartPeriod time.Duration
+	Retries     int // 0 = unlimited
+}
+
+// ParseHealthcheck parses a healthcheck string with optional flag prefix into
+// a HealthcheckConfig. The syntax is:
+//
+// [--interval=<duration>] [--timeout=<duration>] [--start-period=<duration>]
+// [--retries=<n>] <command>
+//
+// Flags must use the form --flag=value. The first token that does not start
+// with "--" and everything after it is treated as the command. Defaults:
+// interval=3s, timeout=10s, start-period=0, retries=0 (unlimited)
+func ParseHealthcheck(s string) (HealthcheckConfig, error) {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return HealthcheckConfig{}, fmt.Errorf("empty healthcheck")
+	}
+
+	hc := HealthcheckConfig{
+		Interval: 3 * time.Second,
+		Timeout:  10 * time.Second,
+	}
+
+	tokens := strings.Fields(s)
+	cmdIndex := -1
+
+	for i, token := range tokens {
+		if !strings.HasPrefix(token, "--") {
+			cmdIndex = i
+			break
+		}
+
+		parts := strings.SplitN(token, "=", 2)
+		if len(parts) != 2 {
+			return HealthcheckConfig{}, fmt.Errorf("malformed flag (missing '='): %q", token)
+		}
+
+		key, val := parts[0], parts[1]
+		switch key {
+		case "--interval":
+			d, err := time.ParseDuration(val)
+			if err != nil {
+				return HealthcheckConfig{}, fmt.Errorf("parse interval: %w", err)
+			}
+			hc.Interval = d
+		case "--timeout":
+			d, err := time.ParseDuration(val)
+			if err != nil {
+				return HealthcheckConfig{}, fmt.Errorf("parse timeout: %w", err)
+			}
+			hc.Timeout = d
+		case "--start-period":
+			d, err := time.ParseDuration(val)
+			if err != nil {
+				return HealthcheckConfig{}, fmt.Errorf("parse start period: %w", err)
+			}
+			hc.StartPeriod = d
+		case "--retries":
+			r, err := strconv.Atoi(val)
+			if err != nil {
+				return HealthcheckConfig{}, fmt.Errorf("parse retries: %w", err)
+			}
+			hc.Retries = r
+		default:
+			return HealthcheckConfig{}, fmt.Errorf("unknown healthcheck flag: %q", token)
+		}
+	}
+
+	if cmdIndex == -1 {
+		return HealthcheckConfig{}, fmt.Errorf("healthcheck has no command")
+	}
+
+	hc.Cmd = strings.Join(tokens[cmdIndex:], " ")
+	return hc, nil
+}