Update the Alpine base image to v3.5

[pascalandy]

Hi,
As alpine:3.5 is now the default image, I think this project shall update all dockerfile from alpine:3.4 to alpine:3.5.

If it's good, I can do the PR :)

Cheers!
Pascal

[tianon]

See also #298 😉

[pascalandy]

Thanks for pointing this out @tianon :)