Using PACKER_LOG=1 causes all the log output to be sent to Stderr. This
change maintains that backward compatility. Anything other than "1" will
be treated as a filename which will have logging appended to that file.
This is useful, for example, to always have debugging available without
cluttering up stdout (and without having to redirect stderr all the time).

Fix typo in packer.go

website: Just fixing a small error in the text.

The sentence was missing a few words.
I've made it read similar to the previous 3 sentences.

provisioner/shell: execute script directly instead of using /bin/sh

/cc @sgirones

/cc @sgirones

builder/virtualbox: Local mode for VBoxGuestAdditions. Provide local path and SHA256

website: Shutdown Command in Examples