Added customized keybase client to manage git repositories

docker/keybaseio-client/Dockerfile

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+FROM keybaseio/client:stable
+
+RUN apt-get update \
+    && apt-get install -y \
+         bindfs \
+         git-lfs \
+    && rm -rf /var/lib/apt/lists/*
+
+# `bindfs` requires the mountpoint to exist.
+ARG MOUNTPOINT='/repos.rw'
+RUN mkdir -p "$MOUNTPOINT"

docker/keybaseio-client/README.md

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+# Keybase client
+
+The image gets `git` so I can use it to manage my repositories on keybase.io.
+
+Can surely be vastly improved.
+
+1. [TL;DR](#tldr)
+2. [Permissions mismatch in the binded directory](#permissions-mismatch-in-the-binded-directory)
+3. [Further readings](#further-readings)
+
+## TL;DR
+
+```sh
+# Build it.
+docker build -t 'michelecereda/keybaseio-client' .
+
+# Start the service.
+# Needs '--privileged' do be able to write to the disk.
+docker run \
+  -d --name 'keybaseio-client' \
+  -e KEYBASE_SERVICE='1' \
+  -e KEYBASE_USERNAME='user' \
+  -e KEYBASE_PAPERKEY='paper key …' \
+  --privileged \
+  -v '/path/to/repos/root:/repos.ro' \
+  'michelecereda/keybaseio-client'
+
+# `bindfs` needs to be run as 'root' to use the '--create-for-user' flag.
+# Need to automate this when the container starts (entrypoint?).
+docker exec -u root 'keybaseio-client' \
+  bindfs \
+    --force-user='keybase' --force-group='keybase' \
+    --create-for-user='keybase' --create-for-group='keybase' \
+    --chown-ignore --chgrp-ignore \
+    '/repos.ro' \
+    '/repos.rw'
+
+# Leverage the service to execute commands.
+docker exec -u 'keybase' 'keybaseio-client' keybase whoami
+docker exec -u 'keybase' 'keybaseio-client' \
+  git clone 'keybase://private/user/repo' '/repos.rw/repo'
+
+# Fix ownership of the new files in the directory (if needed).
+# The container will still be able to see them as its own.
+chown -R 'user':'group' /path/to/repos/root
+```
+
+## Permissions mismatch in the binded directory
+
+Due to continers' nature and user management, there might be a mismatch between the user id in the container and the one of the user owning the repositories directory on the host.
+
+To solve this, I applied Hongli Lai's solution in [Docker and the Host Filesystem Owner Matching Problem]:
+
+```sh
+# Contents of the current test folder.
+$ ls -l
+drwxr-xr-x 1 myuser users   0 Sep 17 20:31 repos
+
+# Start the service.
+$ docker run \
+>   -d --name 'keybaseio-client' \
+>   --privileged \
+>   -v "${PWD}/repos:/repos.ro" \
+>   -e KEYBASE_SERVICE='1' \
+>   -e KEYBASE_USERNAME='user' \
+>   -e KEYBASE_PAPERKEY='paper key …' \
+>   'michelecereda/keybaseio-client'
+e6c550e02e1796cabfd752d8326e3c99d5f3646baa2e9befa34964b94ae67609
+
+# Mount the ro folder in the container to a rw folder I can use.
+$ docker exec -u root 'keybaseio-client' \
+>   bindfs --chown-ignore --chgrp-ignore \
+>     --force-user='keybase' --force-group='keybase' \
+>     --create-for-user='keybase' --create-for-group='keybase' \
+>     '/repos.ro' '/repos.rw'
+
+# Current permissions of the mounted folder on the host.
+$ ls -l repos
+total 0
+-rwx------ 1 myuser users   0 Sep 18 01:21 file.txt
+
+# Current permissions of the mounted folder in the container.
+$ docker exec -u keybase -ti 'keybaseio-client' ls -l '/repos.rw'
+total 0
+-rwx------ 1 keybase keybase   0 Sep 17 23:21 file.txt
+
+# Clone a repository from keybase.
+$ docker exec -u keybase -ti 'keybaseio-client' git clone keybase://private/mek/repo /repos.rw/repo
+Cloning into '/repos.rw/repo'...
+Initializing Keybase... done.
+Syncing with Keybase... done.
+Counting: 10.46 KB... done.
+Cryptographic cloning: (100.00%) 10.46/10.46 KB... done.
+
+# Current permissions of the mounted folder in the container.
+$ sudo docker exec -u keybase -ti 'keybaseio-client' ls -l '/repos.rw'
+total 0
+-rwx------ 1 keybase keybase   0 Sep 17 23:21 file.txt
+drwxr-xr-x 1 keybase keybase 304 Sep 18 08:04 repo
+
+# Current permissions of the mounted folder on the host.
+$ ls -l repos
+total 0
+-rwx------ 1 myuser users   0 Sep 18 01:21 file.txt
+drwxr-xr-x 1 1000   1000  304 Sep 18 08:04 repo
+
+# Fix the permissions on the host.
+$ sudo chown -R 'myuser':'users' 'repos/repo'
+$ ls -l repos
+total 0
+-rwx------ 1 myuser users   0 Sep 18 01:21 file.txt
+drwxr-xr-x 1 myuser users 304 Sep 18 10:04 repo
+
+# Check the permissions of the mounted folder in the container.
+$ docker exec -u keybase -ti 'keybaseio-client' ls -l '/repos.rw'
+total 0
+-rwx------ 1 keybase keybase   0 Sep 17 23:21 file.txt
+drwxr-xr-x 1 keybase keybase 304 Sep 18 08:04 repo
+```
+
+## Further readings
+
+- [keybaseio/client] on DockerHub
+- [Configure the selinux label]
+- [How to create docker volume device/host path] (not used but interesting)
+- [Docker and the Host Filesystem Owner Matching Problem]
+
+[keybaseio/client]: https://hub.docker.com/r/keybaseio/client
+
+[configure the selinux label]: https://docs.docker.com/storage/bind-mounts/#configure-the-selinux-label
+[how to create docker volume device/host path]: https://stackoverflow.com/questions/49950326/how-to-create-docker-volume-device-host-path#49952217
+[docker and the host filesystem owner matching problem]: https://www.fullstaq.com/knowledge-hub/blogs/docker-and-the-host-filesystem-owner-matching-problem