How to get an environment variable value into Dockerfile during “docker build”?

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You should use the ARG directive in your Dockerfile which is meant for this purpose.

The ARG instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg <varname>=<value> flag.

So your Dockerfile will have this line:

ARG request_domain

or if you’d prefer a default value:

ARG request_domain=127.0.0.1

Now you can reference this variable inside your Dockerfile:

ENV request_domain=$request_domain

then you will build your container like so:

$ docker build --build-arg request_domain=mydomain Dockerfile

Note 1: Your image will not build if you have referenced an ARG in your Dockerfile but excluded it in --build-arg.

Note 2: If a user specifies a build argument that was not defined in the Dockerfile, the build outputs a warning:

[Warning] One or more build-args [foo] were not consumed.

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