Lighter FreeSwitch SDK image for CI and GitHub Container Registry

[dariopellegrino00]

CI improvements

The current custom FreeSwitch SDK image used in CI is too heavy, as it includes many unnecessary libraries and packages. We should identify and remove those that are not required for the job.
Use GitHub Container Registry (GHCR) to pull the image instead of recompiling and caching it:

• Even when cached, the image still takes considerable time and disk space.
• It can break when dependencies or libraries change.
• Using GHCR allows for cleaner separation of jobs that need the image, instead of running all checks in a single job (e.g. build, static checks, tests).

[dariopellegrino00]

Implemented in #14, closing issue as completed.