Set the ENTRYPOINT, rather than the CMD. for the Docker build.

This allows us to override the tasks passed to Gradle with our own. For
example, if we wanted to avoid cleaning:

    $ docker run --rm -it -v $PWD:/usr/src/app -v $HOME/.gradle:/root/.gradle sdkman-cli/gradle -x clean

As non-test tasks can now be passed to Gradle, I have changed the tag
of the image to something more general.

Dockerfile

@@ -5,4 +5,4 @@ RUN mkdir -p /usr/src/app
 ADD . /usr/src/app
 WORKDIR /usr/src/app
 
-CMD ["./gradlew"]
+ENTRYPOINT ["./gradlew"]

README.md

@@ -36,12 +36,12 @@ Mac users can add the following line to their `~/.bash_profile` file to set this
 
 You can run the tests in a Docker container to guarantee a clean test environment.
 
-    $ docker build --tag=sdkman-cli/testing .
-    $ docker run --rm -it sdkman-cli/testing
+    $ docker build --tag=sdkman-cli/gradle .
+    $ docker run --rm -it sdkman-cli/gradle
 
 By running the following command, you don't need to wait for downloading Gradle wrapper and other dependencies. The test reports can be found under the local `build` directory.
 
-    $ docker run --rm -it -v $PWD:/usr/src/app -v $HOME/.gradle:/root/.gradle sdkman-cli/testing
+    $ docker run --rm -it -v $PWD:/usr/src/app -v $HOME/.gradle:/root/.gradle sdkman-cli/gradle
 
 ## Running the Server Locally