Damn I Love Docker: Miscellaneous Docker Tips

Table of Contents

• Introduction
• Sync Container and Host Time
• Container Restart Policy
• References

Introduction

While there have been many posts written on this blog discussing the various applications of Docker, there is need to actually discuss some of the “finer” aspects of using docker. In this post we will introduce, and discuss some of the “lesser known” options, commands, and features related to Docker.

Sync Container and Host Time

This is the best way I have found to sync the time between host and container (for logging purposes):¹

-v /etc/localtime:/etc/localtime:ro

Simply mount the volume for your /etc/localtime into the container. This will ensure that both the host machine running the Docker Engine and the containers that will be run by the engine all have the same time.

Container Restart Policy

After enough time working with docker, experimenting with different options and commands, and even running important applications in docker, you will eventually want to know how to “restart” a container automatically. This is all handled with the --restart option.²

For example, I run a dockerized OpenVPN server on my home network, and I want to make sure that it is ALWAYS RUNNING. This makes sense, because I may be in situations where I need to access the VPN, but due to a power failure at some point, the server has been restarted. Without the --restart option set the container running my VPN is not automatically restarted … This would be a serious pain in the &$$, if you catch my drift. But with the --restart option, this can be alleviated like so:

docker create --restart=always  --name=ovpn -v /etc/localtime:/etc/localtime:ro -v $OVPN_DATA:/etc/openvpn -p 1194:1194/udp --cap-add=NET_ADMIN tigerj/rpi-ovpn

To clarify, the above command comes from a previous tutorial on docker OpenVPN³, and if you look towards the beginning of the command you will see the --restart=always option. As you could imagine, this ensures that your container will (per the documentation): “Always restart the container if it stops.” Again, check out the reference² if you would like to know more details or other restart policies.

envsubst

The envsubst command is a tool⁴ that will parse a file and search for shell variables to replace with their corresponding environment variable values and then print the original file with the variable values substituted.

Below is an example NGINX reverse proxy configuration:

$ cat reverse_proxy.tmpl
server {
  listen ${LISTEN_PORT};
  listen [::]:${LISTEN_PORT};

  server_name ${SERVNAME};

  location / {
      proxy_pass ${DOMAIN}:${DOMAIN_PORT};
  }
}

If we set the shell values as follows:

$ LISTEN_PORT=80; SERVNAME=yourdomain.com; DOMAIN=localhost; DOMAIN_PORT=5000

And then run envsubst

$ envsubst < reverse_proxy.tmpl > reverse_proxy.conf

Then we will have a config file with our variables:

$ cat reverse_proxy.conf
server {
  listen 80;
  listen [::]:80;

  server_name yourdomain.com;

  location / {
      proxy_pass localhost:5000;
  }
}

References