упростил схему

.env

@@ -1,5 +1,3 @@
 NPM_IMAGE=jc21/nginx-proxy-manager
 NPM_VERSION=2.11.1
-NPM_HOSTNAME=<CHANGE> #npm
-DOMAIN_NAME=<CHANGE> #example.com
-CA_HOSTNAME=<CHANGE> #ca
+STEP_CA_FQDN=ca.example.com

Dockerfile

@@ -3,10 +3,9 @@ ARG NPM_VERSION
 
 FROM $NPM_IMAGE:$NPM_VERSION
 
-ARG DOMAIN_NAME  
-ARG CA_HOSTNAME
-ENV CA_FQDN=${CA_HOSTNAME}.${DOMAIN_NAME}
+ARG STEP_CA_FQDN  
+ENV STEP_CA_FQDN=${STEP_CA_FQDN}
 
-RUN echo -e "server=https://$CA_FQDN/acme/acme/directory\nno-verify-ssl = true" >> /etc/letsencrypt.ini
+RUN echo -e "server=https://$STEP_CA_FQDN/acme/acme/directory\nno-verify-ssl = true" >> /etc/letsencrypt.ini
 
-RUN sed -i "s\Let's Encrypt\\$CA_FQDN\g" /app/frontend/js/*.js
+RUN sed -i "s\Let's Encrypt\\$STEP_CA_FQDN\g" /app/frontend/js/*.js

docker-compose.yml

@@ -1,18 +1,16 @@
 version: '3.8'
+
 services:
   npm:
     image: '${NPM_IMAGE}-step-ca:${NPM_VERSION}'
     container_name: npm
-    hostname: ${NPM_HOSTNAME}.${DOMAIN_NAME}
     restart: unless-stopped
     build:
       context: ./
       args:
         NPM_IMAGE: ${NPM_IMAGE}
         NPM_VERSION: ${NPM_VERSION}
-        NPM_HOSTNAME: ${NPM_HOSTNAME}
-        DOMAIN_NAME: ${DOMAIN_NAME}
-        CA_HOSTNAME: ${CA_HOSTNAME}
+        STEP_CA_FQDN: ${STEP_CA_FQDN}
     ports:
       - '80:80'
       - '443:443'