Dynamically replace environment variable names by its values inside file

Dockerfile

FROM perl:5-slim

WORKDIR /data
COPY myfile.txt .

RUN perl -pe 's/\$([_A-Z]+)/$ENV{$1}/g'  < myfile.txt > /tmp/myfile.txt
RUN perl -pe 's/\$(\{)?([a-zA-Z_]\w*)(?(1)\})/$ENV{$2}/g' < /tmp/myfile.txt > myfile.txt

# Only substitute variable that are defined perl -pe 's{\$(\{)?(\w+)(?(1)\})}{$ENV{$2} // $&}ge'

main.sh

docker build -t substituter:0.1 .
docker run --rm -v $(pwd):/data substituter:0.1

myfile.txt

Hello ${NAME}

Thank you ${NAME} for sending ${THING}.

I like this $THING
Thank you again $NAME

Best