playground VS OpenWhisk

I've been looking into something similar as well lately. You can actually look at the code for the Go Playground here. I'm currently in the process of reading through the Go Playground code base to do something similar to what you're describing. As one last preface, I agree that just opening up the TCP port on Docker is a bad idea.

Serverless functions are now offered by many cloud providers, as well as having options like OpenFaaS, Knative, Apache's Openwhisk and more from the open source community that run in environments ranging from one server all the way up to globally replicated private clusters.

OpenWhisk - https://openwhisk.apache.org

The serverless functions with Digital Ocean are based on Apache Open Whisk, so the service has additional name space, which need to go into the URL.

The two biggest options are OpenWhisk and OpenFaas. Check out /r/serverless for more options. I'm experimenting currently with OpenFaas as it's the lighter weigh to of the two.

If you meant lambda for cloud functions provided by Amazon then this is open source and free, as long as you host it yourself: https://openwhisk.apache.org/

There are so many of them in big data, e.g. Kafka, Spark, Flink, Delta, Snowplow, Finagle, Deequ, CMAK, OpenWhisk, Snowflake, TheHive, TVM-VTA, etc.

The foundation is Apache OpenWhisk https://github.com/apache/openwhisk with DO specific customization to work with managed databased, log shipping, and other DO specific capabilities. The closest programming model is AWS Lambda in terms of the semantics and execution model.