nginx-proxy-manager/docker/rootfs/etc/cont-init.d/01_s6-secret-init.sh

#!/usr/bin/with-contenv bash
# ref: https://github.com/linuxserver/docker-baseimage-alpine/blob/master/root/etc/cont-init.d/01-envfile

# in s6, environmental variables are written as text files for s6 to monitor
# seach through full-path filenames for files ending in "__FILE"
for FILENAME in $(find /var/run/s6/container_environment/ | grep "__FILE$"); do
    echo "[secret-init] Evaluating ${FILENAME##*/} ..."

    # set SECRETFILE to the contents of the full-path textfile
    SECRETFILE=$(cat ${FILENAME})
    # SECRETFILE=${FILENAME}
    # echo "[secret-init] Set SECRETFILE to ${SECRETFILE}"  # DEBUG - rm for prod!

    # if SECRETFILE exists / is not null
    if [[ -f ${SECRETFILE} ]]; then
        # strip the appended "__FILE" from environmental variable name ...
        STRIPFILE=$(echo ${FILENAME} | sed "s/__FILE//g") 
        # echo "[secret-init] Set STRIPFILE to ${STRIPFILE}"  # DEBUG - rm for prod!
        
        # ... and set value to contents of secretfile
        # since s6 uses text files, this is effectively "export ..."
        printf $(cat ${SECRETFILE}) > ${STRIPFILE}
        # echo "[secret-init] Set ${STRIPFILE##*/} to $(cat ${STRIPFILE})"  # DEBUG - rm for prod!"
        echo "[secret-init] Success! ${STRIPFILE##*/} set from ${FILENAME##*/}"

    else
        echo "[secret-init] cannot find secret in ${FILENAME}"
    fi
done