unoconv: improve gpg reliability

 * gpg key verification occasionally fails due to network issues when attempting to fetch keys. Combat this with two approaches: * Disable ipv6 testing in dirmngr.conf. [1] * Provide a fallback list of keyservers to pull from. [2] [1] usbarmory/usbarmory-debian-base_image#9 (comment) [2] tianon/gosu#39 (comment)

Author: felliott

unoconv/Dockerfile

@@ -33,7 +33,14 @@ RUN apt-get update \
  && apt-get install -y \
  curl \
  gnupg2 \
- && gpg --keyserver pool.sks-keyservers.net --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4 \
+ && mkdir ~/.gnupg && chmod 600 ~/.gnupg && echo "disable-ipv6" >> ~/.gnupg/dirmngr.conf \
+ && for server in hkp://ipv4.pool.sks-keyservers.net:80 \
+ hkp://ha.pool.sks-keyservers.net:80 \
+ hkp://pgp.mit.edu:80 \
+ hkp://keyserver.pgp.com:80 \
+ ; do \
+ gpg --keyserver "$server" --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4 && break || echo "Trying new server..." \
+ ; done \
  && curl -o /usr/local/bin/gosu -SL "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$(dpkg --print-architecture)" \
 	&& curl -o /usr/local/bin/gosu.asc -SL "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$(dpkg --print-architecture).asc" \
 	&& gpg --verify /usr/local/bin/gosu.asc \
@@ -55,7 +62,13 @@ RUN apt-get update \
  && apt-get install -y \
  curl \
  gnupg2 \
- && gpg --keyserver pool.sks-keyservers.net --recv-keys AFEEAEA3 \
+ && for server in hkp://ipv4.pool.sks-keyservers.net:80 \
+ hkp://ha.pool.sks-keyservers.net:80 \
+ hkp://pgp.mit.edu:80 \
+ hkp://keyserver.pgp.com:80 \
+ ; do \
+ gpg --keyserver "$server" --recv-keys AFEEAEA3 && break || echo "Trying new server..." \
+ ; done \
  && curl -SL "$LIBREOFFICE_MIRROR_URL/$LIBREOFFICE_VERSION/deb/x86_64/$LIBREOFFICE_ARCHIVE" -o $LIBREOFFICE_ARCHIVE \
  && curl -SL "$LIBREOFFICE_MIRROR_URL/$LIBREOFFICE_VERSION/deb/x86_64/$LIBREOFFICE_ARCHIVE.asc" -o $LIBREOFFICE_ARCHIVE.asc \
  && gpg --verify "$LIBREOFFICE_ARCHIVE.asc" \