docker-umask-wrapper.sh

#!/bin/bash

# This wrapper script overcomes the problem of docker-produced files in a shared-volume not being
# readable or writeable by the non-root user in the docker host.

# NB: you still need to separately ensure that the newly created files have the
# correct group permission. This can be achieved as follows:
## chmod g+s /srv/octopress-master/{public,config,_deploy,source/_posts}

# Ensures all newly created files will be 775, instead of the debian default of 755.
umask 002

eval "$@"

Dockerfile

ADD docker-umask-wrapper.sh /bin/docker-umask-wrapper.sh
RUN chmod u+x /bin/docker-umask-wrapper.sh

Makefile

docker-run:
  mkdir -p posts config public deploy
  chmod g+s posts config public deploy
  docker run -t -i \
  -v `pwd`/posts:/srv/octopress-master/source/_posts \
  -v `pwd`/config:/srv/octopress-master/config \
  -v `pwd`/public:/srv/octopress-master/public \
  -v `pwd`/deploy:/srv/octopress-master/_deploy \
  -v `dirname $(SSH_AUTH_SOCK)`:`dirname $(SSH_AUTH_SOCK)` -e SSH_AUTH_SOCK=$(SSH_AUTH_SOCK) \
  -p 4000:4000 \
  wujtruj/octopress \
  docker-umask-wrapper.sh $(cmd)