envsubst within docker using build-arg

Question

Is it possible to use envsubst with build-arg during build time of the docker image ? I have a following scenario:

FROM alpine as substitutor
ARG ROOT_PWD
RUN apk add --no-cache envsubst
WORKDIR /app
COPY ./myfile.json .

RUN export ROOT_PASSWORD=${ROOT_PWD}  && \
    envsubst < myfile.json

But it doesn't seem to be working. any ideas ?

Answer 1

I suspect you are using DOCKER_BUILDKIT, in which case by default the output is not shown.

You can either disable DOCKER_BUILDKIT or you can change the progress output.

So given:

$ cat myfile.json
echo "$ROOT_PASSWORD"

and Dockerfile:

FROM alpine as substitutor
ARG ROOT_PWD
RUN apk add --no-cache envsubst
WORKDIR /app
COPY ./myfile.json .

RUN export ROOT_PASSWORD=${ROOT_PWD}  && \
    envsubst < myfile.json

1. Disabling DOCKER_BUILDKIT:

   $ DOCKER_BUILDKIT=0 docker build --no-cache -t foo --build-arg ROOT_PWD=foo . 
   ...
   Step 6/6 : RUN export ROOT_PASSWORD=${ROOT_PWD}  &&     envsubst < myfile.json
   ---> Running in 392f9f557efc
   echo "foo"
   ...
   

2. Using --progress=plain:

   $ docker build --progress=plain --no-cache -t foo --build-arg ROOT_PWD=bar .
   ...
   #9 [5/5] RUN export ROOT_PASSWORD=foob  &&     envsubst < myfile.json
   #9 sha256:11149db43acce77a19337f355e010081c758738bae15daa2c4ddf8405e85c7c7
   #9 0.559 echo "bar"
   #9 DONE 0.6s
   ...
   

Answer 2

Your issue is that you're reading the file myfile.json but you're not writing the output. So you need to change your envsubst command to be redirected to an output file like this:

RUN export ROOT_PASSWORD=${ROOT_PWD} && envsubst < myfile.json > myfile-out.json