feat/Make apt available when basing images off of Unstructured Ubuntu

[jonasheschl]

The build system for the unstructured Ubuntu image seems to remove apt from the image. This is an issue when basing a custom image off of the Unstructured Ubuntu image. In these cases, most consumers will want to install additional packages. With the current Unstructured Ubuntu image, this is not easily possible.

The following Dockerfile for instance, would error out:

FROM downloads.unstructured.io/unstructured-io/unstructured:latest

RUN apt-get install -y clang gcc

[...]

It would be very useful for apt/apt-get to remain available up to the last layer on the Docker image.