add --no-tty flag to gpg call in test Dockerfile (#338)

for some reason, the call without --no-tty fails since a few days

closes #338

Author: beniwohli

tests/Dockerfile

@@ -16,7 +16,7 @@ RUN for server in $(shuf -e ha.pool.sks-keyservers.net \
                             hkp://p80.pool.sks-keyservers.net:80 \
                             keyserver.ubuntu.com \
                             hkp://keyserver.ubuntu.com:80 \
-                            pgp.mit.edu) ; do gpg --keyserver "$server" --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4 && s=0 && break || s=$?; done; (exit $s)
+                            pgp.mit.edu) ; do gpg --no-tty --keyserver "$server" --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4 && s=0 && break || s=$?; done; (exit $s)
 
 RUN curl -o /usr/local/bin/gosu -SL "https://github.com/tianon/gosu/releases/download/1.10/gosu-$(dpkg --print-architecture)" \
     && curl -o /usr/local/bin/gosu.asc -SL "https://github.com/tianon/gosu/releases/download/1.10/gosu-$(dpkg --print-architecture).asc" \