CONTINUAL INTEGRATION

Problem scenario
Sometimes you try to start a Docker container but there is a problem.  For example you try:
docker start <containerID>
But you receive this: "Error response from daemon: Cannot start container <containerID>: failed to create endpoint <name> on network bridge: ip tables failed: iptables --wait -t nat -A DOCKER -p tcp -d 0/0 --dport 80 -j DNAT --to-destination x.x.x.x:80 ! -i docker0: iptables: No chain/target/match by that name."
​
Solution
cd /var/lib/docker/network/files/
ls -lh > /tmp/forposterity
mkdir /tmp/backupdir
mv /var/lib/docker/network/files/ /tmp/backupdir
systemctl restart docker #change this command depending on your distribution and version of Linux to restart Docker services

Problem Scenario
Sometimes the "docker save" command does not work as you would expect.  You run the command on a Docker host, but it does not work.  You might even try the -o flag or the --output=/tmp/destinationFileName.tar option.  But the response may be an surprising error "Cowardly refusing to save to a terminal. Use the -o flag or redirect."  You want to copy a Docker image from one machine to another computer but problems get in your way.  How do you solve this problem so you can transfer a flat file to another server and use the Docker image?  In other words, how do you do the most basic task with Docker, how do you bring a copy of a Docker container to another machine (another virtual server, a different Docker host)?

Solution
Prerequisites
You must have Docker installed.  If you need help installing Docker, see this posting.

Procedures
The redirect in the error is the right clue.  This solution assumes you have Docker installed on the different server. 

From the Docker host, use these three steps to copy a Docker container and place the container on another machine (server or host).

Step #1:  This command should work:
docker save repositoryName:versionName > /tmp/destinationFileName.tar

Alternatively, you could try this command:
docker save ImageName > /tmp/imagename.tar
# You could find the ImageName by running "docker ps -a" and looking at the results.

Step #2:  The above command will save the image as a regularly accessible flat file with the name destinationFileName.tar.  To use it on the destination Docker host, transfer it to that server.  Use scp or sftp to do the file transfer to a different server. 

Step #3:  Then use this command (assuming the location of the file on the destination server is /tmp/) on the different host:  
​docker load < /tmp/destinationFileName.tar

By the way, there are numerous books on Docker.

Goal: You want two Docker containers to use the same file share on the Docker server from time to time.  

Problem/Error blocking goal:  In a Docker container, when you try to change directories to a directory that is on the Docker host (e.g., the docker container was created with the --volume flag), and you get an error "bash: cd <directoryname> permission denied," you have to run two commands to fix it.  An alternative problem is that you can cd into the directory, but when you try to list the files, you get "cannot open directory: Permission denied."  The same solution applies.

Solution:  From the Docker server, run these commands:
     su -c "setenforce 0"
     chcon -Rt svirt_sandbox_file_t </path/to/directoryname>

#where </path/to/directoryname> is the full path of the directory and its name

If you are running CentOS/RHEL/Fedora, you can avoid this problem after a reboot by doing the following on the Docker server itself:
1)  Create /etc/profile.d/custom.sh
2)  Provide these three lines as its contents:
#!/bin/bash
su -c "setenforce 0"
chcon -Rt svirt_sandbox_file_t </path/to/directoryname>

#where </path/to/directoryname> is the full path of the directory and its name

Normally you can enter a Docker container with this command:
docker exec -it <containerID> bash
It is possible you receive an error that says 'exec "bash": executable file not found in $PATH'
The root cause could be related to insufficient disk space on the Docker host.  Delete files or otherwise make room (e.g., add disk space).  Then stop the container with this command: docker stop <containerID>
Finally, restart the container with this command: docker start <containerID>
Now you should be able to enter the container.