Breeze build image or docs behind proxy

Hello all,

**Description**
Add to Breeze commands `build-image` and `build-docs` (needs also in other commands?) a parameter (or more) to specify proxy configurations for the image and docs build.

**Use case / motivation**
Recently I started to contribute to Airflow project, but I had some problems creating and configure the development environment with Breeze. My workstation is behind my company proxy and this creates a problem in the download of the necessary Python packages and the GPG public key during the image build. My workstation is already configured to work with proxy (I configured apt, docker...) but in this case, it didn't work.

I have a similar problem with `./breeze build-docs` command.

**Are you willing to submit a PR?**

I'm not an expert in Breeze or bash script. Applying these 2 changes was enough to be able to build the image with Breeze:

1. scripts/ci/libraries/_build_images.sh

2. scripts/docker/install_mysql.sh

But of course, it will be better to use some global parameters, and I don't know if there are other cases where it is important to set proxy configuration.

Start with the Breeze build-image and build-docs commands, then trace proxy handling in scripts/ci/libraries/_build_images.sh and scripts/docker/install_mysql.sh. Determine whether the same configuration must reach other build commands. Done means proxy settings can be supplied consistently for image and documentation builds, including package and GPG-key downloads.

Written by the indexing model from the issue text.