Reusable containers documentation

[cwansart]

In addition to my question on Slack (https://testcontainers.slack.com/archives/C9EJ7TVT7/p1586615207007900) I created a small documentation showing what reusable containers are and how to use them.

I did take many parts directly from Sergei's pull request #1781 since it describes many parts pretty good. I hope that is okay, otherwise I will re-write those parts.

As requested in the Slack conversation, I added (ALPHA) to the page title and added a box at the top. I think that should be sufficient to make clear, that this is an alpha feature.

There are some places where was I was not completely sure. I added // TODO comments there. Please check them and tell me what you think. I will remove them when they are done.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you believe this is a mistake, please reply to this comment to keep it open. If there isn't one already, a PR to fix or at least reproduce the problem in a test case will always help us get back on track to tackle this.

[stale]

This issue has been automatically closed due to inactivity. We apologise if this is still an active problem for you, and would ask you to re-open the issue if this is the case.

[ghilainm]

Why is this closed? I didn't find the relevant information in documentation. It's all trial and error.

[sporkisfaster]

@rnorth @bsideup @kiview can we get this re-opened and merged to master please? I think a lot of people have use for this and there's no live docs for it currently.

[frankiedrake]

For those who needs to enable container reusage programmaticaly, e.g. for a test/stage tests when you can't modify home directory, you can use
TestcontainersConfiguration.getInstance().updateUserConfig("testcontainers.reuse.enable", "true");
P.S. If this is breaking a low, I'm sorry, let me know :)

[kiview]

@frankiedrake While we would not officially advertise this usage, feel free to use it if at your own risk it works for your use case. Note that this will also result in the .testcontainers.properties file being written/updated.