ARG or ENV which one to use in this case

Question

This could be maybe a trivial question but reading docs for ARG and ENV doesn t put things clear to me  I am building a PHP-FPM container and I want to give the ability for enable disable some extensions on user needs  Would be great if this could be done in the Dockerfile by adding conditionals and passing flags on the build command perhaps but AFAIK is not supported  In my case and my personal approach is to run a small script when container starts  something like the following     bin sh    set -e  RESTART  quot false quot     This script will be placed in  config init  and run when container starts  if     quot  INSTALL XDEBUG quot      quot true quot     then     printf  quot  nInstalling Xdebug     n quot      yum install -y  php71-php-pecl-xdebug     RESTART  quot true quot  fi        if     quot  RESTART quot      quot true quot     then     printf  quot  nRestarting php-fpm     n quot      supervisorctl restart php-fpm fi  exec  quot    quot   This is how my Dockerfile looks like  FROM reynierpm centos7-supervisor ENV TERM xterm       PATH  quot  root  composer vendor bin   PATH  quot        INSTALL COMPOSER  quot false quot        COMPOSER ALLOW SUPERUSER 1       COMPOSER ALLOW XDEBUG 1       COMPOSER DISABLE XDEBUG WARN 1       COMPOSER HOME  quot  root  composer quot        COMPOSER CACHE DIR  quot  root  composer cache quot        SYMFONY INSTALLER  quot false quot        SYMFONY PROJECT  quot false quot        INSTALL XDEBUG  quot false quot        INSTALL MONGO  quot false quot        INSTALL REDIS  quot false quot        INSTALL HTTP REQUEST  quot false quot        INSTALL UPLOAD PROGRESS  quot false quot        INSTALL XATTR  quot false quot   RUN yum install -y https   dl fedoraproject org pub epel epel-release-latest-7 noarch rpm                      https   rpms remirepo net enterprise remi-release-7 rpm RUN yum install -y            yum-utils           git           zip           unzip           nano           wget           php71-php-fpm           php71-php-cli           php71-php-common           php71-php-gd           php71-php-intl           php71-php-json           php71-php-mbstring           php71-php-mcrypt           php71-php-mysqlnd           php71-php-pdo           php71-php-pear           php71-php-xml           php71-pecl-apcu           php71-php-pecl-apfd           php71-php-pecl-memcache           php71-php-pecl-memcached           php71-php-pecl-zip  amp  amp            yum clean all  amp  amp  rm -rf  tmp yum   RUN ln -sfF  opt remi php71 enable  etc profile d php71-paths sh  amp  amp        ln -sfF  opt remi php71 root usr bin  pear pecl phar php php-cgi phpize   usr local bin    amp  amp        mv -f  etc opt remi php71 php ini  etc php ini  amp  amp        ln -s  etc php ini  etc opt remi php71 php ini  amp  amp        rm -rf  etc php d  amp  amp        mv  etc opt remi php71 php d  etc    amp  amp        ln -s  etc php d  etc opt remi php71 php d  COPY container-files   RUN chmod  x  config bootstrap sh WORKDIR  data www EXPOSE 9001  Currently this is working but     If I want to add let s say 20  a random number  of extensions or any other feature that can be enable disable then I will end with 20 non necessary ENV  because Dockerfile doesn t support  env files  definition whose only purpose would be set this flag for let the script knows what to do then      Is this the right way to do it  Should I use ENV for this purpose   I am open to ideas if you have a different approach for achieve this please let me know about it

User · Answer

So if want to set the value of an environment variable to something different for every build then we can pass these values during build time and we don't need to change our docker file every time.

While ENV, once set cannot be overwritten through command line values. So, if we want to have our environment variable to have different values for different builds then we could use ARG and set default values in our docker file. And when we want to overwrite these values then we can do so using --build-args at every build without changing our docker file.

For more details, you can refer this.

User · Answer

From Dockerfile reference    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   The ENV instruction sets the environment variable  lt key gt  to the value  lt value gt   The environment variables set using ENV will persist when a container is run from the resulting image     So if you need build-time customization  ARG is your best choice  If you need run-time customization  to run the same image with different settings   ENV is well-suited   If I want to add let s say 20  a random number  of extensions or any other feature that can be enable disable  Given the number of combinations involved  using ENV to set those features at runtime is best here  But you can combine both by   building an image with a specific ARG using that ARG as an ENV  That is  with a Dockerfile including  ARG var ENV var   var   You can then either build an image with a specific var value at build-time  docker build --build-arg var xxx   or run a container with a specific runtime value  docker run -e var yyy

[docker] ARG or ENV, which one to use in this case?

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