A review of Containerizing Applications

A review of Containerizing Applications

Build a dockerfile

Build a dockerfile. This will instruct docker how to build an image.
This image is a mock python-to-mysql server.
This could be called python-api.Dockerfile.

# START with a base image, here the ubuntu ok
FROM ubuntu

# install deps through "dockerland"
RUN apt-get update
RUN apt-get install python
RUN pip install flask
RUN pip install flask-mysql

# Copy source code from "host" into image
# into a directory at ./opt/source-code
# NOTE: copy . is a bit sloppy but par for course for learners
COPY . ./opt/source-code

# run the api server
ENTRYPOINT FLASK_AP=/opt/source-code/app.py flask run

Dockerfiles Are Instructions and Arguments

Briefly, dockerfiles are a list of "instructions" and "args":

FROM ubuntu - FROM is the instruction, ubuntu is the arg
RUN apt-get update: RUN is the instruction, apt-get update is the arg
etc

Dockerfiles Describe Layers

Each instruction+argument creates a "layer".
Above - 7 instructions, 7 layers.
When Docker builds the image, layers are created and "cached" by docker.
When the same Dockerfile is used to build a new image, docker checks each "layer" with previously-cached layers. When previously cached layers are present, Docker does not do the work of re-creating the image & instead just uses the previously cached layers.

If a dockerfile changes and a new image built, say the above dockerfile adds a new RUN some-other-command after the last RUN command with a new line after line 8, Docker will use the layer created through line 8 and start creating new "layers" with the new command.

Build an Image from the Dockerfile

Build the image using docker.

docker build -f python-api.Dockerfile -t py-api

The output will show the layers being built.
The layers can also be inspected:

docker history my-docker-username/py-api

Check Out Some Info About An Image

The History of the Image

docker image history will show some info about an image:

 docker image history mongo:5.0.2
IMAGE          CREATED         CREATED BY                                      SIZE      COMMENT
31299b956c79   11 months ago   /bin/sh -c #(nop)  CMD ["mongod"]               0B        
<missing>      11 months ago   /bin/sh -c #(nop)  EXPOSE 27017                 0B        
<missing>      11 months ago   /bin/sh -c #(nop)  ENTRYPOINT ["docker-entry…   0B        
<missing>      11 months ago   /bin/sh -c #(nop) COPY file:df3353d9b2c25ef8…   13.2kB    
<missing>      11 months ago   /bin/sh -c #(nop)  VOLUME [/data/db /data/co…   0B        
<missing>      11 months ago   /bin/sh -c mkdir -p /data/db /data/configdb …   0B        
<missing>      11 months ago   /bin/sh -c set -x  && export DEBIAN_FRONTEND…   554MB     
<missing>      11 months ago   /bin/sh -c #(nop)  ENV MONGO_VERSION=5.0.2      0B        
<missing>      11 months ago   /bin/sh -c echo "deb http://$MONGO_REPO/apt/…   72B       
<missing>      11 months ago   /bin/sh -c #(nop)  ENV MONGO_MAJOR=5.0          0B        
<missing>      11 months ago   /bin/sh -c #(nop)  ENV MONGO_PACKAGE=mongodb…   0B        
<missing>      11 months ago   /bin/sh -c #(nop)  ARG MONGO_REPO=repo.mongo…   0B        
<missing>      11 months ago   /bin/sh -c #(nop)  ARG MONGO_PACKAGE=mongodb…   0B        
<missing>      11 months ago   /bin/sh -c set -ex;  export GNUPGHOME="$(mkt…   1.16kB    
<missing>      11 months ago   /bin/sh -c mkdir /docker-entrypoint-initdb.d    0B        
<missing>      11 months ago   /bin/sh -c set -ex;   savedAptMark="$(apt-ma…   14.7MB    
<missing>      11 months ago   /bin/sh -c #(nop)  ENV JSYAML_VERSION=3.13.1    0B        
<missing>      11 months ago   /bin/sh -c #(nop)  ENV GOSU_VERSION=1.12        0B        
<missing>      11 months ago   /bin/sh -c set -eux;  apt-get update;  apt-g…   7.25MB    
<missing>      11 months ago   /bin/sh -c groupadd -r mongodb && useradd -r…   333kB     
<missing>      11 months ago   /bin/sh -c #(nop)  CMD ["bash"]                 0B        
<missing>      11 months ago   /bin/sh -c #(nop) ADD file:cec21619ecbd37b4c…   65.6MB

The Starting OS

docker run -it mongo:5.0.2 cat /etc/os-release

# output ->
NAME="Ubuntu"
VERSION="20.04.3 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.3 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal
Jakes-4:in-depth Jake$ docker run -it mongo:5.0.2 cat /etc/os-release
NAME="Ubuntu"
VERSION="20.04.3 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.3 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal

Send the Image to the public Image Rep

Send the newly-build image into a docker registery. Here, the image will be pushed to the public registry. The image will be named my-docker-acct/py-app.

What Can Be Containerized

dbs
dev tools
os's
browsers
"complete" applications

Tags:

Docker

dockerfile

containers