Dockerfile

ALWAYS FOLLOWS THIS FORMAT: INSTRUCTION ARGUMENT

FROM

• Inherits and sets Base Image (like Base class) or OS

• eg. python:3.6

  • 3.6 is the tag/version

RUN apt-get update

• Runs this linux command and downloads all OS package related updates in the container

COPY ./<your-directory> /<your-directory>

• Copies local folder into the container's folder

  • i.e. ./app in local folder copies to container's /app

• For local folder, the . is the same directory as the Dockerfile

WORKDIR

• Starts container in thie workdir directory

  • i.e.

    • mkdir workdir

    • cd workdir

Note: These 2 ways are equivalent:

COPY ./api /api
WORKDIR /api

WORKDIR /api # makes work directory and cds into it
COPY . . # then you are already in the folder, so you can just use a .

ENV

• Sets environment variable (global variable in shell) in the container

• Will do export MYSQL_USER=ml

CMD

• Command at startup of container (if there is another command in docker run..., this command replaces the one in the Dockerfile)

ENTRYPOINT

• Command at startup of container (if there is another command in docker run..., this command is APPENDED to the one in the Dockerfile)

Using both CMD and ENTRYPOINT

You do this when you want a default command, but the argument can be changed

FROM Ubuntu

ENTRYPOINT ["sleep"]

CMD ["5"]

So here, if no parameter is specified in docker run... , then sleep 5 will run. Otherwise, sleep <argument> will run.

colon :

• This means port

• If you do google:80, just goes to google

  • 80 is the default port for websites

• 22 is the default port for ssh

gunicorn

• -w # Tells you number of CPU cores/threads (1 CPU core handles 1 request at a time)

• -b :#: bind to port # of container

• -t #: timeout in seconds

Formatting

These are all equivalent: Array: RUN/CMD/... [a, b, c, d] Shell: RUN/CMD/... a b c d

Running commands inside a Docker container

# Running bash inside container
$ docker exec -it container_name /bin/bash

# Running command without being inside container
$ docker exec container_name <command>

Caching

Caching when building Docker images can be very useful as it saves a lot of time when installing dependencies.

In what cases do Docker commands not use their cache?

• If the contents that the command is applying itself on changes.

  • e.g. COPY ./example /copy . If ./example is changed in any way, this line in the Dockerfile will not use the cache.

• If anything before the command is installed/changed that wasn't installed/changed before.

  # Dockerfile before
  RUN apt-get update
  COPY ./example /copy
  
  # Dockerfile after
  RUN apt-get update && apt-get -y install cron # A new package is being installed here
  COPY ./example /copy # This line will not use its cache

Running another program before starting container

# The bash script will call a program: e.g. python3 api/example.py
COPY download.sh /usr/local/bin/download.sh
RUN chmod +x /usr/local/bin/download.sh

WORKDIR /
CMD [ "/bin/sh", "-c", "/usr/local/bin/download.sh && gunicorn -w 3 -b :8000 -t 360 --reload api.wsgi:app" ]