delete Dockerfile for testing purposes

We have a pipeline in place to verify pull requests, and all users have the ability to run the tests on their local machines.

Author: jkralik

Dockerfile

@@ -128,15 +128,6 @@ The go-coap provides servers and clients for DTLS, TCP-TLS, UDP, TCP in golang l
 
 [Client](examples/mcast/client/main.go) example.
 
-## Contributing
-
-In order to run the tests that the CI will run locally, the following two commands can be used to build the Docker image and run the tests. When making changes, these are the tests that the CI will run, so please make sure that the tests work locally before committing.
-
-```shell
-docker build . --network=host -t go-coap:build --target build
-docker run --mount type=bind,source="$(pwd)",target=/shared,readonly --network=host go-coap:build go test './...'
-```
-
 ## License
 
 Apache 2.0