don't embed ansi escape codes in appkey

[pfeyz]

given that it's the recommended way to generate the code in the docs, redirecting stdout will produce an appkey that will make bookstack crash when it runs. this will affect anyone who is trying to automate the initial steps of deployment.

$ docker run -it --rm --entrypoint /bin/bash lscr.io/linuxserver/bookstack:latest appkey > app_key
$ xxd app_key 
00000000: 1b5b 3333 6d62 6173 6536 343a 5344 6d4c  .[33mbase64:SDmL
00000010: 7274 4654 5734 6779 4948 6a30 4954 474b  rtFTW4gyIHj0ITGK
00000020: 6a46 764a 374d 7842 5255 7971 7a68 4c4d  jFvJ7MxBRUyqzhLM
00000030: 4e66 6d53 4e4d 593d 1b5b 3339 6d0d 0a    NfmSNMY=.[39m..

i think this is likely the cause of this issue: BookStackApp/BookStack#5289 (comment)

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