CI: replace custom "dockerswarm/dind" image with official docker:dind image

[thaJeztah]

CI in this repository is currently using a custom dockerswarm/dind (https://hub.docker.com/r/dockerswarm/dind) in CI.

The custom image is built from a private repo (https://github.com/docker/dind), using the dockerfile below, which was created when no official docker images existed yet on Docker Hub;

# Dockerfile for building Docker
FROM debian:jessie

# compile and runtime deps
# https://github.com/docker/docker/blob/master/project/PACKAGERS.md#runtime-dependencies
RUN apt-get update && apt-get install -y --no-install-recommends \
        ca-certificates \
        curl \
        iptables \
        procps \
        e2fsprogs \
        xz-utils \
        git \
    && apt-get clean && rm -rf /var/lib/apt/lists/*

ARG VERSION
COPY get_docker.sh /get_docker.sh
RUN bash /get_docker.sh

RUN curl -L -o /dind https://raw.githubusercontent.com/docker/docker/master/hack/dind \
    && chmod +x /dind

VOLUME /var/lib/docker

ENTRYPOINT ["/dind"]

We should replace this image with the official docker:dind image from Docker Hub, so that we no longer have to build the custom image.

Note that the official image is based on alpine, so some changes might be needed, e.g. the Dockerfile has some apt-get lines; https://github.com/docker/swarm/blob/b9091461c559c357e4017b400e24c07517399f4b/test/integration/Dockerfile