feat(config): implement environment variable substitution in configuration file reading

internal/config/config.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"os"
+	"regexp"
 	"strconv"
 	"strings"
 	"sync"
@@ -215,10 +216,23 @@ func (cfg *Config) StartServers(opts ...*StartServersOptions) {
 	}
 }
 
+var envRegex = regexp.MustCompile(`\$\{([^}]+)\}`) // e.g. ${CLOUDFLARE_API_KEY}
+var readFile = os.ReadFile
+
+func (cfg *Config) readConfigFile() ([]byte, error) {
+	data, err := readFile(common.ConfigPath)
+	if err != nil {
+		return nil, err
+	}
+	return envRegex.ReplaceAllFunc(data, func(match []byte) []byte {
+		return strconv.AppendQuote(nil, os.Getenv(string(match[2:len(match)-1])))
+	}), nil
+}
+
 func (cfg *Config) load() gperr.Error {
 	const errMsg = "config load error"
 
-	data, err := os.ReadFile(common.ConfigPath)
+	data, err := cfg.readConfigFile()
 	if err != nil {
 		if os.IsNotExist(err) {
 			log.Warn().Msg("config file not found, using default config")

internal/config/config_test.go

@@ -0,0 +1,38 @@
+package config
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestConfigEnvSubstitution(t *testing.T) {
+	os.Setenv("CLOUDFLARE_AUTH_TOKEN", "test")
+	readFile = func(_ string) ([]byte, error) {
+		return []byte(`
+---
+autocert:
+	email: "test@test.com"
+	domains:
+		- "*.test.com"
+	provider: cloudflare
+	options:
+		auth_token: ${CLOUDFLARE_AUTH_TOKEN}
+`), nil
+	}
+
+	var cfg Config
+	out, err := cfg.readConfigFile()
+	require.NoError(t, err)
+	require.Equal(t, `
+---
+autocert:
+	email: "test@test.com"
+	domains:
+		- "*.test.com"
+	provider: cloudflare
+	options:
+		auth_token: "test"
+`, string(out))
+}