Unable to pass an environment variable to the Dockerfile build process

[gavinharriss]

I'm struggling with an issue where I need to pass an environment variable into the Dockerfile build process. Can anyone point out what I'd doing wrong?

I've created a minimal example to quickly reproduce the issue at https://github.com/gavinharriss/batect-dockerfile-env-issue

One workaround I attempted was to use build_args instead of environment settings, but this also failed.

Any pointers would be greatly appreciated. Thank you in advance as I've been banging my head against this for a few days now.

[charleskorn]

If you'd like to pass an environment variable to the Dockerfile build process, there are a couple of things you need to do:

1. Pass the environment variable as a build arg to the Dockerfile in your Batect config, for example:

   containers:
     aspnet:
       build_directory: . 
       build_args:
          secret_key: $secret_key # Note that the <{...} syntax is for config variables, not environment variables

2. Declare the build arg in your Dockerfile with ARG and then either create an environment variable with the value of the build arg, or use the value directly in subsequent commands:

   ARG secret_key
   ENV secret_key=$secret_key # or eg. RUN do-secret-stuff --key=$secret_key

There's more detailed information about build args in the Docker documentation: https://docs.docker.com/engine/reference/builder/#arg

[gavinharriss]

Thank you, I had tried the ARG approach without success, but I realised when looking at the examples in the link you provided that I had my ARG in the wrong spot.

Originally I had the incorrect approach:

ARG secret_key
FROM mcr.microsoft.com/dotnet/framework/sdk:4.8.1 AS build
ENV SECRET_KEY=$secret_key

But the correct way that works is:

FROM mcr.microsoft.com/dotnet/framework/sdk:4.8.1 AS build
ARG secret_key
ENV SECRET_KEY=$secret_key

Thank you! :)