feat: Allow overriding temporary directory with $TMPDIR env var (#7)

Sometimes /tmp doesn't have enough space. Allow overriding that path with $TMPDIR.

Example error:

{"level":"error","error":"errors encountered while building soci layers; write /tmp/tmp.3943925700: no space left on device; write /tmp/tmp.1788043098: no space left on device","RegistryURL":"<accountid>.dkr.ecr.<region>.amazonaws.com","ImageDigest":"<digest>","time":"2024-09-30T03:22:06Z","message":"SOCI index build error"}

handler.go

@@ -99,11 +99,11 @@ func indexAndPush(ctx context.Context, repo string, digest string, registryUrl s
 	return BuildAndPushSuccessMessage, nil
 }
 
-// Create a temp directory in /tmp
+// Create a temp directory in /tmp or $TMPDIR
 // The directory is prefixed by the Lambda's request id
 func createTempDir(ctx context.Context) (string, error) {
 	log.Info(ctx, "Creating a directory to store images and SOCI artifacts")
-	tempDir, err := os.MkdirTemp("/tmp", "soci") // The temp dir name is prefixed by the request id
+	tempDir, err := os.MkdirTemp("", "soci") // The temp dir name is prefixed by the request id
 	return tempDir, err
 }