Based on PR #367, fixes #356 - massive thanks to @SQLJames and the previous reviewers. Hopefully this PR can push the work the last inch across the line :)

⚠ Work In Progress! I'm not the best at shell scripts, and unfamiliar with Factorio config (I'm just starting out creating a mod), so please check thoroughly! I made this PR as a starting point and haven't tested everything (just that it builds).

TODO

• Verify each variable is not required, and the default loads correctly
• Verify each variable can be set, and overrides the default
• Verify bad variables are safely handled
• README.md - What should the be set in the "Available in" column? https://github.com/factoriotools/factorio-docker/blob/8c2fc392239eeb0f05987adff4cf3f2b40d1a8af/README.md#environment-variables

Summary

docker-entrypoint.sh now has five variables with default values

• SERVER_SETTINGS_FILE - default $CONFIG/server-settings.json
• SERVER_ID_FILE - default $CONFIG/server-id.json
• ADMINLIST_FILE - default $CONFIG/server-adminlist.json
• WHITELIST_FILE - default $CONFIG/server-whitelist.json
• BANLIST_FILE - default $CONFIG/server-banlist.json

Changes from the original PR

• I moved the variables to be next to the initialisation of the other variables (FACTORIO_VOL, SAVE_NAME, ...)
• I used the same pattern of initialisation that the other variables use VAR="${VAR:-default}"
• added a variable for server-settings.json
• I used $CONFIG as a base directory for all of the variables
• I removed the 'FACTORIO_prefix, made all the variables end with_FILE, and made sure each name matched the default filename (e.g. ADMINLISTinstead ofADMIN_LIST`.
• I wasn't sure what the difference between mkdir -p "$BANLIST_FILE" and mkdir -p "$(dirname "$BANLIST_FILE")" - I played it safe and used the pattern that was already in the file. I figured it out - dirname is required so the parent directory of the config file is made. I've added it back in.