Added 3.4.1 version and parametrized gatling version on build

[axelbergh]

Added on Dockerfile:

ARG gatling_version=3.4.1

# gatling version
ENV GATLING_VERSION=$gatling_version

to be able to override gatling version on docker build:

docker build --build-arg gatling_version=3.4.1 github.com/denvazh/gatling

See this post

[axelbergh]

@denvazh could you take a few minutes to review this PR, please?

[nptri]

Hi, did you try your new update dockerfile with newest Gatling ?