Allow custom Entrypoint

It seems that there are some parts missing compared to what is actually running at AWS CodeBuild?

E.g. if I build this image and push to ECR, then DinD does not work. DinD works perfectly fine though when running locally.

Doing a `ps -e` inside codebuild shows me that PID 1 is:

```
1 root 0:00 /codebuild/bootstrap/linux-bootstrap -zipName=linux-binaries.zip -url=https://codefactory-eu-west-1-prod-default-build-agent-executor.s3.amazonaws.com/linux-binaries.zip
```

meaning the whole entrypoint (including dockerd-entrypoint.sh) are being overridden by codebuild. It seems like the codebuild-boostrapper has either a detection or is relying on something else that is not included in this project to make dind work.

Start by tracing how the CodeBuild linux-bootstrap entrypoint at PID 1 overrides dockerd-entrypoint.sh, comparing the behavior of this image with the managed AWS CodeBuild environment. Determine where custom entrypoints are selected or replaced and verify that DinD works when the intended entrypoint is honored in CodeBuild.

Written by the indexing model from the issue text.