Summary - Environment

Combine ENV, ARG and -e to write a configurable image.

Summary: environment

In this group:

-e NAME=value gives a variable at run time; -e NAME takes it from the host.
ENV adds a permanent default to the image; -e overrides it.
ARG is valid only in the build; it's filled with --build-arg and before FROM can choose the base image.
--env-file loads many variables from a file; secret files aren't put in the repository.
WORKDIR is the directory for the following instructions; RUN cd isn't persistent.

docker build --build-arg SURUM=1.5.0 -t api:1.5.0 .
docker run -d --env-file app.env -e LOG_LEVEL=debug api:1.5.0

Exercise Build and run settings

Write two commands: build api:1.5.0 from the current folder with the build argument SURUM=1.5.0; then start a container named api in the background with the app.env file and LOG_LEVEL=debug.

Show example solution

docker build --build-arg SURUM=1.5.0 -t api:1.5.0 .
docker run -d --name api --env-file app.env -e LOG_LEVEL=debug api:1.5.0

Test yourself

What is done to also read a build argument's value in the running container?
- Nothing; ARG stays automatically
- -e ARG is written
- It isn't possible
- It is passed to ENV with ENV NAME=$NAME
Show answer

It is passed to ENV with ENV NAME=$NAME — ARG disappears after the build; ENV stays in the image.

Override the env file value with a command option.

docker run -d --env-file app.env ____ LOG_LEVEL=debug api:1.0

Show answer

docker run -d --env-file app.env -e LOG_LEVEL=debug api:1.0

Match.

-e

Run-time variable
ENV

Image default
ARG

Build variable
--env-file

Load from file

Next lesson: Compose design

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