Motivation
----------

It looks like building images with containertool on Linux fails downloading the
base image `swift:slim` from dockerhub.

Modifications
-------------

Update end to end and integration workflows to test both `scratch` and `swift:slim`
base images.

Result
------

End-to-End Tests will now fail in release configuration.

Test Plan
---------

Other tests are not affected by this.

Docker Hub now redirects pulls for swift:slim to an S3 bucked behind
CloudFlare. URLSession on macOS follows the redirect correctly, but
URLSession on Linux incorrectly includes an Authorization header
which S3 rejects.

This commit adds a URLSession delegate which removes the Authorization
header when following a redirect.

…nux (#167)

Motivation
----------

In #164, @herzi reported a problem building container images based on
`swift:slim` on Linux. The root cause of the problem is that Docker Hub
now redirects pulls for `swift:slim` to an S3 bucked behind CloudFlare.
URLSession on macOS follows the redirect correctly, but URLSession on
Linux incorrectly includes an `Authorization` header which S3 rejects.

Modifications
-------------

Add a URLSession delegate which removes the `Authorization` header when
following a redirect.

Result
------

Pulling `swift:slim` from Docker Hub works again.

Fixes: #166 

Test Plan
---------

* #164 extended the end-to-end tests to use `swift:slim` as well as
`scratch`, demonstrating the problem. The extended tests now pass.
* All existing tests continue to pass.