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_ADMIN capability. 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。
