Update Android Dockerfile to install Docker

[simonrozsival]

We need to be able to build a Docker image during the Android build process so that we can run a gRPC testing server (see dotnet/runtime#73060).

cc @directhex

[mthalman]

How are containers from this image intended to be created?

This is a docker-in-docker scenario which requires specific configuration to work properly. It either needs to be run with the Docker socket volume-mounted or with the privileged option. I see that the image is being referenced to be used as a container job: https://github.com/dotnet/runtime/blob/cfe26ecb3b677a506711878c19586ddd06117d0e/eng/pipelines/common/platform-matrix.yml#L209-L211. Does that even support either of those methods?

[simonrozsival]

The /var/run/docker.sock volume should be mounted correctly (https://dev.azure.com/dnceng/public/_build/results?buildId=1911472&view=logs&j=0657b2d6-71c1-5c98-76b8-e3cb824cf6bd&t=7162708e-c8ea-47a5-bc28-5a1a3986e307&l=77) so docker-in-docker should work. @directhex tested it locally so he can fill in more details.

[mthalman]

Ah, thanks for the pointer. Yes, that should work then.

[mthalman]

I can merge this but the Ubuntu pipeline is currently broken: #644 (comment). So that needs to get resolved before an image for this can be built.

[mthalman]

Image is published now. Tag: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-android-20220803135138-8fcaabc