Docker (Legacy)

06 Apr 2017

getting started
tips
- debugging inside container
- ping another container
troubleshooting
- Cannot connect to the Docker daemon. Is the docker daemon running on this host?
- lookup registry-1.docker.io on 192.168.65.1:53: no such host

getting started

docker deals with containers, docker-compose deals with services.

codeschool

http://campus.codeschool.com/courses/try-docker/level/1/section/1/containers--images

container (linux container) - isolated environment that can run code while sharing a single operating system

run container and execute command inside running container:

$ docker container run -p 80:80 httpd:2.4
$ docker container ls
$ docker container exec <container-name> du -mh
$ docker container exec -it <container-name> /bin/bash

image - a blueprint for creating a container

pre-built images are available in Docker Hub. custom image can be built using Dockerfile (which contains instructions to create custom image).

Dockerfile example:
```
FROM httpd:2.4
EXPOSE 80
RUN apt-get update
RUN apt-get install -y fortunes
LABEL maintainer="moby-dock@example.com"
```
build custom image and run container using that image:
```
$ docker image build -t web-server:1.0 .
$ docker image ls
$ docker container run -d -p 80:80 web-server:1.0
```

volume - exposes files on the host to the container (it’s like mounting host directory inside the container)

copy files to container:

$ docker container cp page.html <container-name>:/usr/local/apache2/htdocs/

copy files to custom image in Dockerfile:

RUN ...
COPY page.html /usr/local/apache2/htdocs/

use data volume to share data:

$ docker run -d -p 80:80 -v /my-files:/usr/local/apache2/htdocs web-server:1.0

docker-compose

http://stackoverflow.com/a/35585573/3632318

.docker-compose.yml file lists services along with their images, commands and volumes (= mounted directories).

service is used to run command within specified image, service is run by 1+ containers.

for docker-compose commands to work .docker-compose.yml file must be present in current or one of parent directories.

docker-compose up

create and start containers for all services (pull required Docker images from Docker hub if necessary).

use -d option to do it in the background:

docker-compose up -d
docker-compose start/stop/restart app

start/stop/restart specified service.

docker-compose ps

list containers (name, command, state, ports):

       Name                      Command               State           Ports
-------------------------------------------------------------------------------------
server_app_1          rackup -o 0.0.0.0 -p 4321        Up      0.0.0.0:4321->4321/tcp
server_db_1           docker-entrypoint.sh postgres    Up      5432/tcp

use -a option to show all containers (only running containers are shown by default):

docker ps -a

docker-compose logs app

view output from all containers or from containers of specified service.

app above is a service name.

use -f option to follow log output (= tail -f):

docker-compose logs -f app

use -t option to show timestamps:

docker-compose logs -t app
docker-compose run app sh

run a one-off command on a service - service name must be specified.

if command (like sh) is not specified service command is run.

there are no -t and -i options just like for docker run to start interactive session - container is run in attached mode by default.

use -d option to run container (and command inside it) in the background:

docker-compose run -d app sh

use -p option to publish container’s port(s) to the host:

docker-compose run -p <host_port>:<container_port> app sh
docker-compose exec app sh

execute a command in a running container - service name must be specified.

command MUST be specified.

there are no -t and -i options just like for docker exec to start interactive session - command is run in attached mode by default.

use -d option to run command in the background:

docker-compose exec -d app service motd start

NOTE: using -d option doesn’t make sense for all commands: e.g. for command above it doesn’t matter if -d option is used or not at all - the result is the same. or else using -d option for sh command causes all output to disappear from screen (yet control is not returned to the host).

docker

docker images/docker image ls

list images.

docker ps

list containers (container id, image, command, created, status, ports, names):

CONTAINER ID   IMAGE       COMMAND                  PORTS                    NAMES
732******500   v***n/n**   "rackup -o 0.0.0.0..."   0.0.0.0:4321->4321/tcp   server_app_1
e0e******c7c   postgres    "docker-entrypoint..."   5432/tcp                 server_db_1

note that CREATED and STATUS columns have been removed from output for the sake of readability.

docker logs server_app_1

same as docker-compose logs app but for container instead of service.
docker build -t v***n/n** .

build image from Dockerfile (the latter must be present in CWD). it’s necessary to rebuild the whole image when you make any changes that must be persisted (for app service: when you modify ruby source code).
docker run -ti v***n/n** sh

NOTE: any changes made within image (like installing packages, etc.) are lost when container exits (except changes in mounted volume)!

run command in a new container - image name must be specified.

if command (like sh) is not specified default command is run (argument passed to CMD in base image’s Dockerfile).

use -t option to allocate pseudo-TTY (without it no prompt is displayed).

use -i option to make session interactive (without it session exits after the first user input inside container).

use -v to bind mount a volume inside container:

docker run -ti -v <local_dir>:<remote_dir> v***n/n** sh
docker exec -ti 732******500 /bin/bash

run command in a running container - container id from docker ps output must be specified (unlike image name for docker run).

command MUST be specified.

use -i and -t options in the same way as for docker run.

tips

debugging inside container

this is just one way and probably not the most efficient one:

set breakpoint by making byebug call somewhere in your code
stop app container

docker-compose stop app
run shell command in app container

docker-compose run -p 4321:1234 app sh
start rack server manually

rackup -o 0.0.0.0 -p 1234
refresh the page that would allow you to hit breakpoint
start debugging inside running container as you usually do
exit from running container when debugging session is over

ping another container

https://docs.docker.com/v17.09/engine/userguide/networking

Docker creates a network between all containers managed by the same daemon so it’s possible to access other containers by their names.

say, we have 2 services - app and db:

$ docker-compose exec app sh
# ping db
PING db (172.18.XXX.XXX): 56 data bytes
64 bytes from 172.18.XXX.XXX: icmp_seq=0 ttl=64 time=0.359 ms

troubleshooting

Cannot connect to the Docker daemon. Is the docker daemon running on this host?

solution

just install Docker app via brew cask.

it’s possible to install all the necessary docker tools (boot2docker, docker, docker-compose, etc.) manually but I didn’t manage to configure them all in reasonable time. so it’s better to install Docker app - it does all the heavy lifting for you.

$ brew cask install docker

don’t forget to run Docker app after installation (otherwise docker command will not be available in terminal).

lookup registry-1.docker.io on 192.168.65.1:53: no such host

when running docker run hello-world command host to pull hello-world image from is not found.