How to split the nginx config across multiple lines?

You might need to consider variable nesting that will look something like this:

set $PKP '';
set $PKP '${PKP}pin-sha256="X3pGTSOuJeEVw989IJ/cEtXUEmy52zs1TZQrU06KUKg=";'; # current ECDSA
set $PKP '${PKP}pin-sha256="MHJYVThihUrJcxW6wcqyOISTXIsInsdj3xK8QrZbHec=";'; # current RSA (nginx 1.11.0)
set $PKP '${PKP}pin-sha256="isi41AizREkLvvft0IRW4u3XMFR2Yg7bvrF7padyCJg=";';  # Backup ECDSA 1
set $PKP '${PKP}pin-sha256="I/bAACUzdYEFNw2ZKRaypOyYvvOtqBzg21g9a5WVClg=";';  # Backup ECDSA 2
set $PKP '${PKP}pin-sha256="Y4/Gxyck5JLLnC/zWHtSHfNljuMbOJi6dRQuRJTgYdo=";'; # backup RSA 1
set $PKP '${PKP}pin-sha256="/oCVQg3nP3DroGpFdAbaiYzenycUftqrH3LAyaIal2g=";';  # Backup RSA 2

add_header Public-Key-Pins $PKP;