Fix gpg tty error

Seems like debian's GPG version now expects a TTY and only works without 
when it's explicitly definied.

This patch adds the `--no-tty` option to the gpg call. `--batch` seems 
to include it and this way doesn't need it. This should fix the error 
during docker build.

Additional information:
https://github.com/nodejs/docker-node/issues/922
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=913614

debian/Dockerfile

@@ -22,7 +22,7 @@ RUN set -ex; \
     \
 # verify the signature
     export GNUPGHOME="$(mktemp -d)"; \
-    gpg --import /tmp/gosu.key; \
+    gpg --no-tty --import /tmp/gosu.key; \
     gpg --batch --verify /usr/local/bin/gosu.asc /usr/local/bin/gosu; \
     rm -rf "$GNUPGHOME" /usr/local/bin/gosu.asc; \
     \