Error when trying to use "docker build"

[Rodrigo Morales <me@rodrigomorales.site>]

I'm using GuixSD and I'm learning how to use Docker. So, I want to use
Docker daemon under Guix.

I have added =docker-service-type= to my system configuration and
reconfigure my system.

I have created the following =Dockerfile=.

#+BEGIN_SRC sh
cat ~/e/foo/Dockerfile
#+END_SRC

#+RESULTS:
#+begin_example
FROM alpine:3.4

RUN apk update
RUN apk add vim
RUN apk add curl
#+end_example

When I try to use =docker build=, I get the following error:

#+BEGIN_SRC text
$ docker build -t my/docker-sample:1.0 .
Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Post "http://%2Fvar%2Frun%2Fdocker.sock/v1.24/build?buildargs=%7B%7D&cachefrom=%5B%5D&cgroupparent=&cpuperiod=0&cpuquota=0&cpusetcpus=&cpusetmems=&cpushares=0&dockerfile=Dockerfile&labels=%7B%7D&memory=0&memswap=0&networkmode=default&rm=1&shmsize=0&t=my%2Fdocker-sample%3A1.0&target=&ulimits=null&version=1": dial unix /var/run/docker.sock: connect: permission denied
$ echo $?
1
#+END_SRC

I thought that adding my user to the =docker= group might solve the
problem, but it didn't (or at least not in my system). I also thought
that restarting the docker daemon using =sudo herd restart dockerd=
might solve the problem, but it didn't.

How to troubleshoot the error "Got permission denied while trying to
connect to the Docker daemon socket at unix:"?