fix: avoid checking for the reaper out of its mutex

[mdelapenya]

What does this PR do?

This PR removes redudant checks for getting the test session ID, simply delegating it to the sessionID global value, not reading it from the reaper (which already uses the very same value).

Why is it important?

Having a nil-check for the reaperInstance to get the test sessionID introduced a race condition, as it's only set to not nil under a code block that is protected by a Mutex. Therefore, the check caused problems instead of benefits. I added that check intentionally, thinking that always reading from the reaper could be a good idea. But it was not.

Related issues

• Caused by feat: define test session semantics #1513
• Closes [Bug]: Data Race accessing reaper instance for docker containers #1691

How to test this PR

This should demonstrate the bug (before this fix) and the fix:

go test -timeout 600s -run ^Test_BuildImageWithContexts$ . -race -count=1 -v

Also running all reaper tests:

go test -timeout 600s -run "^(TestContainerStartsWithoutTheReaper|TestContainerStartsWithTheReaper|TestContainerStopWithReaper|TestContainerTerminationWithReaper|TestContainerTerminationWithoutReaper|Test_NewReaper|Test_ReaperForNetwork|Test_ReaperReusedIfHealthy|TestReaper_reuseItFromOtherTestProgramUsingDocker)$" github.com/testcontainers/testcontainers-go -count=1 -race -v

[netlify]

✅ Deploy Preview for testcontainers-go ready!

Name	Link
🔨 Latest commit	3a14ffe
🔍 Latest deploy log	https://app.netlify.com/sites/testcontainers-go/deploys/65140ea012db14000858ec4c
😎 Deploy Preview	https://deploy-preview-1692--testcontainers-go.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[mdelapenya]

As a follow up, I think we could include the -race flag whenever running tests in the CI. I think it's a good practice that could help us identify race conditions the soonest.

I think I'll do it in a separate PR.