Use the -f flag instead of -m flag for readlink

[Guiorgy]

The -m flag isn't as portable as I thought. According to Mark Altmann, the -m is not available on older Mac systems, and may not be available on BSD.

The -f flag should be more universal.

I'll test to make sure all things work as they did, until then I'll leave this as a draft.

Edit: Here's a list of things I tested:

• Normal paths (not containing any links) work as before, so no issues here.
• If the destination path (e.g. test4/mosquitto_config.tar.gz) contains a link to itself (e.g. test4 -> test4), readlink with -m returns the path to the link (e.g. /home/user/directory/test4), while with -f it returns an empty string. Either case results in a Docker error, though different ones.
• Paths containing 1 valid link work as intended (e.g. 'test5 -> test3', test5/mosquitto_config.tar.gz, both -m and -f correctly resolve to /home/user/directory/test3)
• Using paths with last link pointing to a nonexistent directory works in both cases (e.g. running rm -rf test3 in the above case before exporting), though this will create that missing directory
• Paths with multiple links work the same in both cases (e.g. test6 -> test6.1 and test6.1 -> test3 when destination is test6/bitwarden_data.tar.gz)
• If the destination contains a link to a nonexistent directory and the target is a subdirectory, -m works but -ffails (e.g.test7 -> test3and destination istest3/test/mosquitto_config.tar.gz, but test3 doesnt exist (rm -rf test3), then -mreturns the correct path/home/user/directory/test3/test, but -f` return an empty string)

Overall, the main difference, from what I can tell, is that -f will faill if a link in the middle of the destination path points to an invalid object, wheres -m works, which was the reason of using it in the first place:

-m, --canonicalize-missing
canonicalize by following every symlink in every component of the given name recursively, without requirements on components existence

If we use -f, might be worth erroring out with a better message when readlink returns an empty string. I'll add that if you agree with that.