* gnu/system/file-systems.scm (%control-groups): Add "pids".
* gnu/services/docker.scm (docker-shepherd-service): Resolve a TODO.
This has allowed me to make a specific configuration of nsjail work.
* gnu/packages/linux.scm (singularity)[source](snippet): Change file
name of setuid helpers in libexec/cli/*.exec.
[arguments]: Remove "--disable-suid".
* gnu/services/docker.scm (%singularity-activation): New variable.
(singularity-setuid-programs): New procedure.
(singularity-service-type): New variable.
* gnu/tests/singularity.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* doc/guix.texi (Miscellaneous Services): Document it.
The Docker proxy enables inter-container and outside-to-container loopback,
and is required by the Docker registry server.
* gnu/services/docker.scm (docker-configuration)[proxy,
enable-proxy?]: Add fields.
(docker-shepherd-service): Use them.
(serialize-boolean): New function.
