Configurable default startup timeout

[kubamarchwicki]

Custom configuration let users override some default properties if your environment requires that. Most of the properties are to mitigate various environment constraints users might have (and let them define the workaround globally in the .testcontainers.properties). For example, timing out images pulls or disable startup checks.

One of the local constraints might be tests startup timing out due. While I'd like to keep the more aggressive timeouts for the CI builds (60 seconds), the test performance on the developers' laptop might be limited. Therefore a possibility relax the default startup timeout globally will make the local build more repeatable.

This is target for a more integration / blackbox testing scenarious, when multiple containers are setup to emulate the deployment environment.

Testing evidence This is a backwards-compatible change. Existing tests pass. The feature is covered by other tests.