Docker is not listed as a requirement for building from source

[erichaagdev]

It is not mentioned in the README that Docker is a requirement for building from source. By default, Docker is required by the com.felipefzdz.gradle.shellcheck plugin. The plugin can also be configured to not require Docker, in which case it will download a compatible binary using a configured installer. We should consider this as another possible option.

We have two possible options:

1. Add having Docker running as a requirement to the Build from source section.
2. Configure the com.felipefzdz.gradle.shellcheck plugin to not use Docker, and instead use an installer.

[etiennestuder]

Let's do #1 for now.

[etiennestuder]

PR is here.

[etiennestuder]

Fixed.

[etiennestuder]

//cc @DPUkyle