Only rebuild if custom_gdb_script contents changed across runs

[0xricksanchez]

Setting manual toggles for rebuilds in the config.ini that forces rebuilding (which is required) for changes in io/scripts/gdb_script to take effect inside the debugger container is tedious. A PR for this should introduce a "watcher" that automatically rebuilds the debugger (basically only executes the COPY at the end of the respective dockerfile) IFF the contents of the aforementioned gdb script changed compared to the prior run. If force_rebuild is toggled on for the debugger in config.ini rebuilding always takes place.