Added post about setting the user for builds inside Docker containers.

content/docker_uid.rst

+Building inside a Docker container with the correct user
+########################################################
+
+:date: 2017-11-26
+:summary: Building inside a Docker container with the correct user
+
+Lately I've been using Docker container as clean, easily portable and easily
+removable build environments. In those cases the image contains the needed build
+tools and the project is mounted to a volume inside the container. The artifacts
+are then built inside the container but are placed inside the volume. However
+a small problem arises, the artifacts (and whatever other files are created,
+like cache) are owned by the default user, :code:`root`, making editing or
+removing said files less straightforward.
+
+The trivial solution
+--------------------
+
+The trivial solution is to run the container with the correct user id, like so
+
+.. code:: shell
+
+    uid="$(id -u)"
+    gid="$(id -g)"
+    docker run -v "$PWD:/volume" --user "$uid:$gid" buildimage make
+
+I personally find it a tiresome after the 3rd time I had to rebuild the
+project because I forgot to specify the uid and gid and it's a (low) barrier
+to entry for new users.
+
+A better solution
+-----------------
+
+The solution I've come up with is this small script that sets the uid and gid
+values to those of the owner and group for the volume and then execute the
+commands.
+
+.. code:: shell
+
+    #!/bin/sh
+    set -eu
+    command -v sudo > /dev/null || { echo "Can't find sudo, exiting."; exit 1; }
+    uid="$(stat . --format '%u')"
+    gid="$(stat . --format '%g')"
+    groupadd --force --non-unique --gid "$gid" builder
+    useradd --non-unique --gid "$gid" --home-dir /volume --no-create-home --shell /bin/sh builder
+    sudo -Eu "#$uid" -g "#$gid" -- "$@"
+
+The script is also available for `download
+<https://www.shore.co.il/blog/static/runas>`_. The only dependency is
+:code:`sudo`. You can download it and check to your VCS and incorporate it into
+your Dockerfile, or download it via the :code:`ADD` directive, like so:
+
+.. code:: shell
+
+    FROM debian:stable
+    RUN DEBIAN_FRONTEND=noninteractive apt-get update && \
+        DEBIAN_FRONTEND=noninteractive apt-get install -y sudo build-essential
+    ADD [ "https://www.shore.co.il/blog/static/runas", "/entrypoint"]
+    ENTRYPOINT [ "/bin/sh", "/entrypoint" ]
+    VOLUME /volume
+    WORKDIR /volume
+    ENV HOME /volume
+
+And then finally, to build run
+
+.. code:: shell
+
+    docker run -v "$PWD:/volume" buildimage make

content/static/runas

+#!/bin/sh
+set -eu
+command -v sudo > /dev/null || { echo "Can't find sudo, exiting."; exit 1; }
+uid="$(stat . --format '%u')"
+gid="$(stat . --format '%g')"
+sudo -Eu "#$uid" -g "#$gid" -- "$@"