docs: add decision record about using Testcontainers (#3255)

docs/developer/decision-records/2023-06-30-allow-use-of-testcontainers/README.md

@@ -0,0 +1,23 @@
+# Allow and regulate the usage of Testcontainers
+
+## Decision
+
+Using Testcontainers is henceforth allowed, and it is the preferred way to integration-test with third-party software
+components.
+
+## Rationale
+
+Testcontainers are a self-contained way to run tests against real third-party software products, such as Postgres. Up
+until now the EDC guideline was to separate test code from infrastructure, but this rule is now relaxed in that
+integration tests may now use Testcontainers.
+
+However, this does _not_ mean that we should use component tests or integration tests for everything. Or: unit tests are
+still the preferred way to test software components, they should continue to make up the largest part of our test code
+base.
+
+## Approach
+
+- Every test that uses a Testcontainer must be annotated either with `@ComponentTest` or another suitable integration
+  test annotation. So when tests are run locally on developers' machines by default only unit tests are executed.
+- New integration/component tests should use Testcontainers
+- Existing component/integration tests are gradually converted over to using Testcontainers

docs/developer/decision-records/README.md

@@ -65,3 +65,4 @@
 - [2023-06-05-validation-engine](2023-06-05-validation-engine)
 - [2023-06-19-change-of-github-labels](2023-06-19-new-issue-triage-process/)
 - [2023-06-19-onboarding-contributors](2023-06-19-onboarding-contributors/)
+- [2023-06-30-allow-use-of-testcontainers](2023-06-30-allow-use-of-testcontainers/)

docs/developer/testing.md

@@ -92,16 +92,15 @@ can also be supplied via system properties.
 There is no one-size-fits-all guideline whether to perform setup tasks in the `@BeforeAll` or `@BeforeEach`, it will
 depend on the concrete system you're using. As a general rule of thumb long-running one-time setup should be done in
 the `@BeforeAll` so as not to extend the run-time of the test unnecessarily. In contrast, in most cases it is **not**
-advisable to deploy/provision the external system itself in either of those methods. In other words, provisioning a
-CosmosDB or spinning up a Postgres docker container directly from test code should generally be avoided, because it will
-introduce code that has nothing to do with the test and may cause security problems (privilege escalation through the
-Docker API), etc.
-
-This does not at all discourage the use of external test environments like containers, rather, the external system
-should be deployed in the CI script (e.g. through Github's `services` feature), or there might even be a dedicated test
-instance running continuously, e.g. a CosmosDB test instance in Azure. In the latter case we need to be careful to avoid
-conflicts (e.g. database names) when multiple test runners access that system simultaneously and to properly clean-up
-any residue before and after the test.
+advisable to deploy/provision the external system itself in either of those methods. In other words, manually
+provisioning a CosmosDB or spinning up a Postgres docker container directly from test code should generally be avoided,
+because it will introduce code that has nothing to do with the test and may cause security problems (privilege
+escalation through the Docker API), etc.
+
+Specifically, if possible all external system should be deployed using [Testcontainers](https://testcontainers.com/).
+Alternatively, in special situations there might be a dedicated test instance running continuously, e.g. a CosmosDB test
+instance in Azure. In the latter case please be careful to avoid conflicts (e.g. database names) when multiple test
+runners access that system simultaneously and to properly clean up any residue before and after the test.
 
 ### Running them locally