feat: add --env-file flag to develop.sh (#25621)

Adds `--env-file` to `scripts/develop.sh` to allow reading environment 
from a given file. This makes it easier to configure things like external 
auth providers, access URLs, and other dev-time settings without 
exporting a wall of environment variables in every shell session.

> Generated with [Coder Agents](https://coder.com/agents)

scripts/develop/main.go

@@ -27,6 +27,7 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/joho/godotenv"
 	"golang.org/x/sync/errgroup"
 	"golang.org/x/xerrors"
 
@@ -69,6 +70,24 @@ const (
 )
 
 func main() {
+	// Pre-parse --env-file before serpent runs so that variables from
+	// the file are visible to serpent's Env-tag resolution for other
+	// options. The flag is also registered in the serpent OptionSet
+	// below for --help discoverability.
+	envFile, err := parseEnvFileFlag()
+	if err != nil {
+		_, _ = fmt.Fprintf(os.Stderr, "develop: %v\n", err)
+		os.Exit(1)
+	}
+	if envFile != "" {
+		n, err := loadEnvFile(envFile)
+		if err != nil {
+			_, _ = fmt.Fprintf(os.Stderr, "develop: error loading env file %s: %v\n", envFile, err)
+			os.Exit(1)
+		}
+		_, _ = fmt.Fprintf(os.Stderr, "develop: loaded %d variable(s) from %s\n", n, envFile)
+	}
+
 	var cfg devConfig
 
 	cmd := &serpent.Command{
@@ -182,6 +201,12 @@ func main() {
 				Description: "Accept changed migration files and update tracking. Use when you've manually fixed the DB to match the new migrations.",
 				Value:       serpent.BoolOf(&cfg.dbContinue),
 			},
+			{
+				Flag:        "env-file",
+				Env:         "CODER_DEV_ENV_FILE",
+				Description: "Path to a .env file to load before starting. Variables in the file do not override existing environment variables. Note: unquoted and double-quoted values undergo $VAR expansion against other entries in the same file (not the process environment); use single quotes for literal dollar signs.",
+				Value:       serpent.StringOf(&cfg.envFile),
+			},
 		},
 		Handler: func(inv *serpent.Invocation) error {
 			cfg.serverExtraArgs = inv.Args
@@ -198,7 +223,7 @@ func main() {
 		},
 	}
 
-	err := cmd.Invoke(os.Args[1:]...).WithOS().Run()
+	err = cmd.Invoke(os.Args[1:]...).WithOS().Run()
 	if err != nil {
 		_, _ = fmt.Fprintf(os.Stderr, "error: %v\n", err)
 		os.Exit(1)
@@ -223,6 +248,9 @@ type devConfig struct {
 	dbRollback        bool
 	dbReset           bool
 	dbContinue        bool
+	// envFile is populated by serpent for --help output; actual loading
+	// uses parseEnvFileFlag() before serpent runs.
+	envFile           string
 	projectRoot       string
 	binaryPath        string
 	configDir         string
@@ -435,8 +463,46 @@ func (c *devConfig) cmd(ctx context.Context, bin string, args ...string) *exec.C
 	return cmd
 }
 
-// filterEnv returns env with any variables whose key matches
-// exclude removed.
+// parseEnvFileFlag extracts the --env-file value from os.Args and
+// CODER_DEV_ENV_FILE before serpent runs, so that loaded variables
+// are visible to serpent's Env-tag resolution for other options.
+func parseEnvFileFlag() (string, error) {
+	for i, arg := range os.Args[1:] {
+		if arg == "--env-file" {
+			if i+2 >= len(os.Args) {
+				return "", xerrors.New("--env-file requires a value")
+			}
+			return os.Args[i+2], nil
+		}
+		if v, ok := strings.CutPrefix(arg, "--env-file="); ok {
+			return v, nil
+		}
+	}
+	return os.Getenv("CODER_DEV_ENV_FILE"), nil
+}
+
+// loadEnvFile reads the file at path using godotenv and sets any variables
+// not already present in the process environment. It returns the number of
+// variables set.
+func loadEnvFile(path string) (int, error) {
+	vars, err := godotenv.Read(path)
+	if err != nil {
+		return 0, err
+	}
+	var n int
+	for key, val := range vars {
+		if _, exists := os.LookupEnv(key); exists {
+			continue
+		}
+		if err := os.Setenv(key, val); err != nil {
+			return n, err
+		}
+		n++
+	}
+	return n, nil
+}
+
+// filterEnv returns env with any variables whose key matches exclude removed.
 func filterEnv(env []string, exclude ...string) []string {
 	out := make([]string, 0, len(env))
 	for _, e := range env {