Add support for gatling 3.0

[CucumisSativus]

First of all, thanks for you awesome work.

Recently I updated my tests to gatling 3.0 and to still use them with your image i needed those changes. Hopefully they will be useful for others :)

libc6-compat is required since gatling tries to load libnetty, which fails, because there is no libcrypt.so.1 in the system

[soedar]

Suggested change

	ENV GATLING_VERSION 3.0.0-RC4
	ENV GATLING_VERSION 3.0.0

Since the stable version was just released, maybe we can use the stable version instead?

[soedar]

I think it would be better to move the installation of the libgc-compact dependency here

[CucumisSativus]

So since 3.0.0 is released I changed the name of the directory, added mention about 3.0.0 to readme and updated link to the latest release

[guikcd]

Is now redirected to https://gatling.io/docs/current/

[CucumisSativus]

Good point! I will put there https://gatling.io/docs/3.0/cheat-sheet/ to make sure it will be still valid even after release of new gatling version. Thanks for pointing that out

[guikcd]

Is now redirected to https://gatling.io/docs/current/cheat-sheet/

[guikcd]

Also gatling is now compatible with Java 9, 10 and 11(see gatling/gatling#3359)

But the openjdk image seems to have dropped alpine support and have now a slim version:

• 8-jdk-alpine 73 MB
• 10-jre-slim 105 MB
• 10-jdk-slim 301 MB

If the switch to more modern images is done, the first instruction of packages need to be changed to use apt instead of apk, for example:

FROM openjdk:11-jre-slim
[...]
RUN apt update && apt -y install wget unzip && \
  rm -rf /var/cache/apt/* /var/lib/apt/lists/* && \
[...]

I've used the gatling default provided simulation: gatling.sh --simulation computerdatabase.BasicSimulation --run-description test

[CucumisSativus]

@soedar what do you think about changing the base image? If you think that update is needed, should we do it in this PR or create a separate one for this?

[soedar]

@CucumisSativus I'm not sure actually, and I do not have merge permission to the repository. Ping @denvazh what do you think?

[CucumisSativus]

@soedar sorry my mistake. Let's ping @denvazh for the second time, just in case ;)

[denvazh]

@CucumisSativus @soedar thank you for pull request and sorry for the delay. I had a lot of stuff on my plate, but I was almost on time to prepare release for rc1 to 3 (#20), thus I will still keep them (and also include 4 while at it) and also merge this PR on top of these changes.

[denvazh]

@CucumisSativus @guikcd for the base java image I think it makes more sense to merge this PR as-is and release and then work on switch.
Personally, I don't really care which base version is used. Alpine was used mainly because of its small image size.
In any case, I have created an issue regarding the base image switch: #21