ECR tasks should have the option to logout on completion?

**Is your feature request related to a problem? Please describe.**

Using the configured AWS Service Connection credentials, the ECR tasks (push and pull) will perform a `docker login` which results in credentials being cached in the docker config of the agent user at `~/.docker/config.json`. No `logout` is subsequently performed. For self-hosted agents, which may not be ephemeral, subsequent executions of unrelated pipelines can use these cached credentials to perform ECR operations. Aside from potentially destructive operations, some docker tasks integrating with ECR which don't use the AWS-provided ECR Push/Pull tasks may behave unpredictably depending on whether a previous pipeline using the ECR Push/Pull tasks has been executed.

**Describe the solution you'd like**

Ideally the ECR Push/Pull tasks could do a `docker logout` in a post-job execution step at the end of the pipeline execution. Some considerations though:

* There could be multiple ECR tasks in a pipeline. Would each one perform a `logout`? Is this an idempotent operation, i.e. do subsequent `logout`s complete without an exception? The post-job execution step could parse the docker `config.json` to determine if a logout is required.
* Do some customers have maintenance processes to log their agent accounts in to ECR? Having the ECR tasks perform a `logout` could disrupt pipelines that depend on these processes. The ECR tasks could track whether they had to perform a `docker login` and only `logout` if they were responsible, though this doesn't look easy to track ([link](https://stackoverflow.com/a/36023944/112196)). I think I'm less concerned here as I would say having cached credentials on disk only for the duration of the pipeline execution would be a better security practice and customers could look at an alternative solution like [amazon-ecr-credential-helper](https://github.com/awslabs/amazon-ecr-credential-helper).

**Describe alternatives you've considered**

Having our own custom process injected into the pipelines to perform a `docker logout` at the end of the pipeline execution.

**Additional context**

Azure DevOps Server 2019.1.1 with self-host Azure Pipeline Agents v2.168.2

Start with the ECR Push/Pull tasks, Docker login behavior, and the ~/.docker/config.json path described in the issue. Review Azure Pipelines post-job execution and decide how multiple tasks and pre-existing credentials should be handled. Done requires an agreed logout policy that prevents unintended credential removal and cleans up credentials created by the task.

Written by the indexing model from the issue text.