由于以下几个原因，Docker的官方CentOS镜像中没有提供systemd服务：
- systemd requires the
CAP_SYS_ADMINcapability. This means running docker with--privileged. Not good for a base image. - systemd requires access to the cgroups filesystem.
- systemd has a number of unit files that don’t matter in a container, and they cause errors if they’re not removed
但在可控环境下，我们还是希望使用systemd来管理我们的服务，如何开启systemd呢？
首先创建一个Dockerfile用来创建systemd的base image：
FROM centos:latest
MAINTAINER feisky
RUN yum -y install systemd systemd-libs
#RUN yum -y update
RUN yum clean all;
(cd /lib/systemd/system/sysinit.target.wants/; for i in *;
do [ $i == systemd-tmpfiles-setup.service ] || rm -f $i; done);
rm -f /lib/systemd/system/multi-user.target.wants/*;
rm -f /etc/systemd/system/*.wants/*;
rm -f /lib/systemd/system/local-fs.target.wants/*;
rm -f /lib/systemd/system/sockets.target.wants/*udev*;
rm -f /lib/systemd/system/sockets.target.wants/*initctl*;
rm -f /lib/systemd/system/basic.target.wants/*;
rm -f /lib/systemd/system/anaconda.target.wants/*;
VOLUME [ "/sys/fs/cgroup" ]
CMD ["/usr/sbin/init"]
执行 docker build --rm -t centos:systemd .来创建一个systemd base image:
[root@fei ~]# docker build --rm -t centos:systemd . Sending build context to Docker daemon 490 MB Sending build context to Docker daemon Step 0 : FROM centos:latest ---> b157b77b1a65 Step 1 : MAINTAINER feisky ---> Using cache ---> 4add1a12ff67 Step 2 : RUN yum -y install systemd systemd-libs ---> Using cache ---> 2bc5274f7dd2 Step 3 : RUN yum clean all; (cd /lib/systemd/system/sysinit.target.wants/; for i in *; do [ $i == systemd-tmpfiles-setup.service ] || rm -f $i; done); rm -f /lib/systemd/system/multi-user.target.wants/*;rm -f /etc/systemd/system/*.wants/*;rm -f /lib/systemd/system/local-fs.target.wants/*; rm -f /lib/systemd/system/sockets.target.wants/*udev*; rm -f /lib/systemd/system/sockets.target.wants/*initctl*; rm -f /lib/systemd/system/basic.target.wants/*;rm -f /lib/systemd/system/anaconda.target.wants/*; ---> Running in d132481218d7 Loaded plugins: fastestmirror Cleaning repos: base extras updates Cleaning up everything Cleaning up list of fastest mirrors ---> 7a9492819396 Removing intermediate container d132481218d7 Step 4 : VOLUME /sys/fs/cgroup ---> Running in d5a93c8ee296 ---> 10b06676ea4d Removing intermediate container d5a93c8ee296 Step 5 : CMD /usr/sbin/init ---> Running in 6ab276a1974b ---> b519e838367f Removing intermediate container 6ab276a1974b Successfully built b519e838367f
下面就可以基于该镜像来创建http服务的镜像了，当然这儿就需要另外一个Dockerfile：
FROM centos:systemd
MAINTAINER feisky
RUN yum -y install httpd; systemctl enable httpd.service
RUN yum -y install mariadb-server mariadb; systemctl enable mariadb.service
RUN yum clean all
EXPOSE 80
CMD ["/usr/sbin/init"]
执行docker build -t http_server .来创建http镜像。
镜像创建好后可以通过下面的命令来启动http docker（注意需要--privileged）：
docker run --privileged -d -v /sys/fs/cgroup:/sys/fs/cgroup:ro -h server.http.local -p 8081:80 -t http_server
通过下面的命令可以进入docker的shell（其中3df7d8c59f1e为容器ID）：
nsenter --mount --uts --ipc --net --pid --target $(docker inspect --format "{{.State.Pid}}" "605bae76456d")
或者可以通过下面的命令来在容器内执行命令：
# docker exec 605bae76456d ps -ef
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 07:33 ? 00:00:00 /usr/sbin/init
root 15 1 0 07:33 ? 00:00:00 /usr/lib/systemd/systemd-journald
root 16 1 0 07:33 ? 00:00:00 /usr/sbin/httpd -DFOREGROUND
apache 18 16 0 07:33 ? 00:00:00 /usr/sbin/httpd -DFOREGROUND
apache 19 16 0 07:33 ? 00:00:00 /usr/sbin/httpd -DFOREGROUND
apache 20 16 0 07:33 ? 00:00:00 /usr/sbin/httpd -DFOREGROUND
apache 21 16 0 07:33 ? 00:00:00 /usr/sbin/httpd -DFOREGROUND
apache 22 16 0 07:33 ? 00:00:00 /usr/sbin/httpd -DFOREGROUND
dbus 187 1 0 07:41 ? 00:00:00 /bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation
MySQL 248 1 0 07:41 ? 00:00:00 /bin/sh /usr/bin/mysqld_safe --basedir=/usr
mysql 404 248 1 07:41 ? 00:00:00 /usr/libexec/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib64/mysql/plugin --log-error=/var/log/mariadb/mariadb.log --pid-file=/var/run/mariadb/mariadb.pid --socket=/var/lib/mysql/mysql.sock
root 458 0 0 07:41 ? 00:00:00 ps -ef
访问服务器的8081端口发现Apache正常启动起来了。
