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Minor fixes and improvements to the Vagrantfile
Please do not use a temporary file here (which would be better created using e.g. `os.tempnam()` anyway) but write/pipe the `decoded_key` into the `apt-key(8)` sub-process using the `stdin` argument and  `StringIO` or similar. Note that one can use a dash (`-`) character as input file path in order to refer to `/dev/stdin` explicitely.