How can I fix bad substitution error inside docker-compose file?

How to run a /bin/bash for loop inside docker-compose file?
It's saying bad substitution error, and I'm not sure if there is another character to escape.

environment:
      domains: (x.com,y.com)

entrypoint:
      - /bin/sh
      - -c
      - |
        curl -s https://raw.githubusercontent.com/certbot/certbot/master/certbot-nginx/certbot_nginx/_internal/tls_configs/options-ssl-nginx.conf > "/etc/letsencrypt/options-ssl-nginx.conf"        echo $$email
        curl -s https://raw.githubusercontent.com/certbot/certbot/master/certbot/certbot/ssl-dhparams.pem > "/etc/letsencrypt/ssl-dhparams.pem"
        ls -la /etc/letsencrypt/
        echo $$domains
        for domain in $${domains[@]}; do
          echo $$domain
        done

In the future, please include enough of your docker-compose.yaml for us to actually reproduce the problem. I think I know what the problem is, but because your question doesn't show what image you're using it's just an educated guess.

Your entrypoint script is using Bash-specific features (specifically, arrays). While on many Linux distributions, /bin/sh is a symlink to /bin/bash, this isn't universally true. On Debian-derived distributions, including Ubuntu, /bin/sh is a more traditional Bourne-shell implementation without the extra features available in Bash.

If use fedora:38 in your docker-compose.yaml, in which /bin/sh is actually Bash, it runs without errors (although it doesn't perform as you intend); the output of the loop is:

example_1  | (x.com,y.com)

On the other hand, if I use ubuntu:22.04, I get the error you've described in your question:

example_1  | /bin/sh: 5: Bad substitution

To get things working, we first need to deal with logic error -- as we saw above, when using Bash, the script runs without an error, but it doesn't actually treat your domains variable as an array.

There are two problems: first, Bash array syntax uses whitespace, rather than commas, to separate items; and second, you can't treat an environment variable as an array in any case. But that's okay, because we don't need an array variable at all! If we write the docker-compose.yaml file like this:

services:
  example:
    image: ubuntu:22.04
    environment:
      domains: "x.com y.com"
    entrypoint:
      - /bin/sh
      - -c
      - |
        for domain in $$domains; do
          echo $$domain
        done

Then the output is:

x.com
y.com

Which is what we want. This doesn't require any bash-specific syntax, which means you can safely use /bin/sh regardless of what distribution you're using.