Update VSTS agent version to 4.255.0 in Dockerfile

Author: btungut

Dockerfile

@@ -4,7 +4,7 @@ ARG ARG_UBUNTU_BASE_IMAGE_TAG="20.04"
 FROM ${ARG_UBUNTU_BASE_IMAGE}:${ARG_UBUNTU_BASE_IMAGE_TAG}
 WORKDIR /azp
 ARG ARG_TARGETARCH=linux-x64
-ARG ARG_VSTS_AGENT_VERSION=4.251.0
+ARG ARG_VSTS_AGENT_VERSION=4.255.0
 
 
 # To make it easier for build and release pipelines to run apt-get,