Use same agent image version as in go.mod

.buildkite/steps/agent.sh

@@ -7,8 +7,12 @@ apk add --update-cache --no-progress skopeo
 
 repo="docker.io/buildkite/agent"
 
+echo --- Awking go.mod for agent version
+agent_version="$(awk '/github\.com\/buildkite\/agent\/v3/ { print $2 }' go.mod | cut -c 2- )"
+echo "Using agent version ${agent_version} as image tag"
+
 echo --- :docker: Inspecting agent docker image manifest
-digest=$(skopeo inspect "docker://${repo}:stable" --format {{.Digest}})
+digest=$(skopeo inspect "docker://${repo}:${agent_version}" --format {{.Digest}})
 
 echo Choosing image "${repo}@${digest}"
 buildkite-agent meta-data set agent-image "${repo}@${digest}"