Get environment variable value in Dockerfile

Question

I m building a container for a ruby app  My app s configuration is contained within environment variables  loaded inside the app with dotenv    One of those configuration variables is the public ip of the app  which is used internally to make links  I need to add a dnsmasq entry pointing this ip to 127 0 0 1 inside the container  so it can fetch the app s links as if it were not containerized   I m therefore trying to set an ENV in my Dockerfile which would pass an environment variable to the container   I tried a few things   ENV REQUEST DOMAIN  REQUEST DOMAIN ENV REQUEST DOMAIN  REQUEST DOMAIN    Everything passes the  REQUEST DOMAIN  string instead of the value of the environment variable though  Is there a way to pass environment variables values from the host machine to the container

User · Answer

If you just want to find and replace all environment variables   ExampleEnvVar  in a Dockerfile then build it this would work   envsubst  lt   path to Dockerfile   docker build -t myDockerImage   -f -

User · Answer

add -e key for passing environment variables to container   example     MYSQLHOSTIP   sudo docker inspect -format      NetworkSettings IPAddress      MYSQL CONRAINER ID    sudo docker run -e DBIP  MYSQLHOSTIP -i -t myimage  bin bash  root 87f235949a13    echo  DBIP 172 17 0 2

User · Answer

Load environment variables from a file you create at runtime   export MYVAR  my var outside  cat  gt  build env sh  lt  lt EOF MYVAR   MYVAR  EOF       then in the Dockerfile  ADD build  build RUN  build test sh   where test sh loads MYVAR from env sh     bin bash    build env sh echo  MYVAR  gt   tmp testfile

User · Answer

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  lt varname gt   lt value gt  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

User · Answer

An alternative using envsubst without losing the ability to use commands like COPY or ADD  and without using intermediate files would be to use Bash s Process Substitution   docker build -f  lt  envsubst  lt  Dockerfile  -t my-target

User · Answer

So you can do  cat Dockerfile   envsubst   docker build -t my-target -  Then have a Dockerfile with something like   ENV MY ENV VAR  MY ENV VAR   I guess there might be a problem with some special characters  but this works for most cases at least

[docker] Get environment variable value in Dockerfile

Examples related to docker