Bake debugging requirements into the nightly image for ease of debugging

[DL6ER]

Description

See title, the instructions are in taken from https://deploy-preview-338--pihole-docs.netlify.app/ftldns/gdb/

Motivation and Context

Related to pi-hole/FTL#2112 (comment)

How Has This Been Tested?

Locally build container, confirmed gdb is working. The image increases quite significantly in size which is why I am not suggesting this for any production, just the hard development tag. I added the installation rather early to benefit from Docker's cache so changes due to a more recent FTL binary, etc., do not need to download much.

$ docker images
REPOSITORY           TAG               IMAGE ID       CREATED          SIZE
<none>               nightly           59b8682afb79   7 seconds ago    144MB
<none>               local             c888fe0ab5a6   36 seconds ago   91.2MB

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.