Added DLC_SPACE_AGE environment variable

[Fank]

This pull request includes updates to the README.md file for better formatting and the addition of a new environment variable, as well as new scripts to manage DLC mods in the Docker setup.

Documentation Updates:

• README.md: Improved table formatting for environment variables and added a new variable DLC_SPACE_AGE to enable or disable DLC mods. [1] [2]

Docker Script Enhancements:

• docker/files/docker-dlc.sh: Added a new script to enable or disable DLC mods based on the DLC_SPACE_AGE environment variable.
• docker/files/docker-entrypoint.sh: Updated the entry point script to call the new docker-dlc.sh script, ensuring DLC mods are configured at startup.

Fixes #510
Related #m-chandler/factorio-spot-pricing#40

[github-actions]

[shellcheck] _{reported by reviewdog 🐶}
Use -n instead of ! -z. SC2236

factorio-docker/docker/files/docker-entrypoint.sh

Line 80 in f52ea10

if [[ ! -z "$PRESET" ]]; then