Cannot evaluate go template for env

[zhaoyao91]

I try to specify LOGSPOUT_STREAM={{.InstanceID}}-{{.Name}} in my compose file to deploy logspout in swarm cluster, it complains that

Updating service logspout_logspout (id: y14jnec1bcszr62zwu1d96ba3)
failed to update service logspout_logspout: Error response from daemon: rpc error: code = InvalidArgument desc = expanding env failed: expanding env "LOGSPOUT_STREAM={{.InstanceID}}-{{.Name}}": template: expansion:1:2: executing "expansion" at <.InstanceID>: can't evaluate field InstanceID in type *template.Context