[flagd] migrate e2e tests to use test containers for container image starts

[aepfli]

Within the Java, Python, and JS SDK, we have already moved to testcontainers.org for testing with the test-harness. This reduces the pain and effort, as the containers are started as part of the tests rather than an extra CI step and/or locally an additional docker command.

Within our ci files:
https://github.com/open-feature/go-sdk-contrib/blob/main/.github/workflows/ci.yaml#L46-L56

See: https://golang.testcontainers.org/ how to integrate testcontainers