I recently came across the issue that I had to build a Docker container with a NextJS app inside that was relying on a environment variable to set the domain for Plausible.io.
The tricky thing was that I was building the container once and deploying it to multiple pods with different environment configs.

Technically docker can respect a .env file when it is run with the option --env-file like so:

docker run --rm --env-file .env -p 3000:3000 -d YOUR_CONTAINER_TAG:prod

So here comes the issue...

But since the environment was set during build time inside the container and NextJS has automatic site optimiziation and builds pre-rendered pages for server-side render during build, the environment variables from the Docker build run were baked into the image and it did not care about the .env file for server logic.

A small bash script to the rescue

I am using Envault to manage environment configs and since the image is being build with a Jenkins pipeline, the pipeline pulls the correct .env file from Envault each time.
Inside the Dockerfile, change the vars to be this:

ARG DB_USER
ARG DD_SVC_NAME
ENV DB_USER=$DB_USER
ENV DD_SVC_NAME=$ARG DD_SVC_NAME

This means the build command needs to contain --build-arg paramters for each variable.
To read those from the .env file via bash, we can use this:

$(for i in `cat .env`; do out+="--build-arg $i " ; done; echo $out;out="")

The docker command looks like this:

docker build -f Dockerfile -t MYTAG:prod $(for i in `cat .env`; do out+="--build-arg $i " ; done; echo $out;out="") .

Bonus round: makefile

Since I love Makefiles, there is a small pitfall how to include the bash loop into the command, so here is my Makefile for reference:

SHELL := /bin/bash
...
# this line will set the build args from env file
DECONARGS = $(shell echo "$$(for i in `cat .env`; do out+="--build-arg $$i " ; done; echo $$out;out="")")
GEN_ARGS = $(eval BARGS=$(DECONARGS))

.PHONY: dist-prod
dist-prod:
    @echo "Running docker build for PROD ..."
        $(GEN_ARGS)
    docker build -f Dockerfile -t $(TAG_SHA) $(BARGS) .
...