This is a simple Docker image that provides the envsubst executable which can be used when expanding environment variables in strings or files.
docker container run --rm -it -e var=value bhgedigital/envsubst sh -c "echo This container has \$var | envsubst"
output:
This container has value
docker container run --rm -it -e var=value -v $(pwd):/wd bhgedigital/envsubst sh -c "envsubst < /wd/file.txt > file-subst.txt" && cat file-subst.txt
output:
This container has value