numeric attribute as environment variable in ContainerDefinition

[frankwese]

I am setting up an MemoryDB cluster and a ECSFargate service. The service should access the MemoryDB. I therefore try to populate the ClusterEndpointAddress and ClusterEndpointPort as ENV variables in the ContainerDefinition of the service.
If I do this:

environment: {
  REDIS_PORT: `redisCluster.cluster.attrClusterEndpointPort.toString(10),
}

CDK creates a CF template:

    "ContainerDefinitions": [
     {
      "Environment": [
       {
        "Name": "REDIS_PORT",
        "Value": {
         "Fn::GetAtt": [
          "MerchantsRedisClustermemorydb803CD209",
          "ClusterEndpoint.Port"
         ]
        }
       }
      

which doesn't work at deployment, because the port is retrieved as a number:
#/ContainerDefinitions/0/Environment/0/Value: expected type: String, found: Integer

If I try to use a template string:

environment: {
  REDIS_PORT: `"${redisCluster.cluster.attrClusterEndpointPort.toString(10)}"`,
}

CDK produces a CF template like:

"ContainerDefinitions": [
     {
      "Environment": [
       {
        "Name": "REDIS_PORT",
        "Value": {
         "Fn::Join": [
          "",
          [
           "\"",
           {
            "Fn::GetAtt": [
             "MerchantsRedisClustermemorydb803CD209",
             "ClusterEndpoint.Port"
            ]
           },
           "\""
          ]
         ]
        }
       }
      ],       

This fails with the error message:

Template error: every Fn::Join object requires two parameters, 
(1) a string delimiter and 
(2) a list of strings to be joined or a function that returns a list of strings (such as Fn::GetAZs) to be joined.

Which is in my opinion erroneous, because there both parameters are present.
This also happens when I try to concat the string:

environment: {
  REDIS_PORT: '"' + redisCluster.cluster.attrClusterEndpointPort.toString(10) + '"',
}

Update:
If I use a literal

environment: {
  REDIS_PORT: Number(5555).toString(10),
}

it produces a correct template:

 "ContainerDefinitions": [
     {
      "Environment": [
       {
        "Name": "REDIS_PORT",
        "Value": "5555"
       }
      ],

Trying to use this "trick" with the CfnAttributes

        REDIS_PORT: Number(redisCluster.cluster.attrClusterEndpointPort).toString(10),

leads to the same template as if I used the attribute directly.

Does anybody have an idea how I can pass a numeric value from CfnAttributes as ENV vars in a ContainerDefinition?