feat: Allow loading of environment variables from file (#20)

* feat: allow loading of environment variables from file

* Panic if a file for an environment variable cannot be read

* Use log.Fatalf + os.Exit instead of panic

* fix: remove supurfluous call to os.Exit()

---------

Co-authored-by: adaexec <nixos-git.s1pht@simplelogin.com>
Co-authored-by: Gabe Farrell <90876006+gabehf@users.noreply.github.com>

cmd/api/main.go

@@ -3,6 +3,8 @@ package main
 import (
 	"fmt"
 	"os"
+	"strings"
+	"log"
 
 	"github.com/gabehf/koito/engine"
 )
@@ -11,7 +13,7 @@ var Version = "dev"
 
 func main() {
 	if err := engine.Run(
-		os.Getenv,
+		readEnvOrFile,
 		os.Stdout,
 		Version,
 	); err != nil {
@@ -19,3 +21,23 @@ func main() {
 		os.Exit(1)
 	}
 }
+
+func readEnvOrFile(envName string) string {
+	envContent := os.Getenv(envName)
+
+	if envContent == "" {
+		filename := os.Getenv(envName + "_FILE")
+
+		if filename != "" {
+			b, err := os.ReadFile(filename)
+
+			if err != nil {
+				log.Fatalf("Failed to load file for %s_FILE (%s): %s", envName, filename, err)
+			}
+
+			envContent = strings.TrimSpace(string(b))
+		}
+	}
+
+	return envContent
+}