Adding support for reading docker context

[jjbustamante]

Summary

This PR adds support to Pack for reading the docker HOST, to be used during the application build, from the current docker context saved at the home user directory.

Output

Before

When running pack build on Linux with Docker desktop installed, the following error was thrown:

ERROR: failed to build: failed to fetch builder image 'gcr.io/buildpacks/builder:latest': 
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

Initially, this error could be workaround by:

• Creating a symlink like sudo ln -s $HOME/.docker/desktop/docker.sock /var/run/docker.sock
• Setting the DOCKER_HOST=unix://$HOME/.docker/desktop/docker.sock /var/run/docker.sock environment variable

After

Currently my docker context configuration is:

> docker context list
NAME                TYPE                DESCRIPTION                               DOCKER ENDPOINT                                     KUBERNETES ENDPOINT   ORCHESTRATOR
default             moby                Current DOCKER_HOST based configuration   unix:///var/run/docker.sock
desktop-linux *     moby                Docker Desktop                            unix:///Users/jbustamante/.docker/run/docker.sock

I used to have the symlink but I removed it, now running pack build, pack will read from my local docker configuration file that desktop-liinux context is being used and then read the endpoint host from there and use it during the build, removing the previous error

looking for docker configuration file at: /Users/jbustamante/.docker
using docker context 'desktop-linux' with endpoint = 'unix:///Users/jbustamante/.docker/run/docker.sock'
Builder paketobuildpacks/builder:base is untrusted
As a result, the phases of the lifecycle which require root access will be run in separate trusted ephemeral containers.
For more information, see https://medium.com/buildpacks/faster-more-secure-builds-with-pack-0-11-0-4d0c633ca619
Pulling image index.docker.io/paketobuildpacks/builder:base
.... 

[exporter] Saving test-image:latest...
[exporter] *** Images (71f626605c5e):
[exporter]       test-image:latest
[exporter]
[exporter] *** Image ID: 71f626605c5e6b4e6883748395c589f1cec7ce8d4fe03c0703fedef67bab2a8b
[exporter] Reading buildpack directory: /layers/paketo-buildpacks_nginx
[exporter] Reading buildpack directory item: launch.toml
[exporter] Reading buildpack directory item: nginx.sbom.cdx.json
[exporter] Reading buildpack directory item: nginx.sbom.spdx.json
[exporter] Reading buildpack directory item: nginx.sbom.syft.json
[exporter] Reading buildpack directory item: nginx.toml
Successfully built image test-image:latest

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

Related

Resolves #1759
Resolves #1830