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")