ci: error if FRONTEND_VERSION not found

[alxndrsn]

Re #2042 (comment)

Adding -e isn't enough to cause a helpful failure when FRONTEND_VERSION is missing from docker-compose.yml because:

1. it doesn't fail, despite bash -e, because the jq command runs inside the echo call, which succeeds, and
2. if the intention is for the failure to be easy to understand, jq -e won't help on its own

What has been done to verify that this works as intended?

• checked behaviour locally
• tested first invocation
• tested second invocation

Why is this the best possible solution? Were any other approaches considered?

Here's what failure looks like now:

[extract-frontend-version.sh] Ensuring .env exists for docker compose...
[extract-frontend-version.sh] Reading FRONTEND_VERSION from docker-compose.yml...
time="2026-07-07T03:33:30Z" level=warning msg="The \"DOMAIN\" variable is not set. Defaulting to a blank string."
time="2026-07-07T03:33:30Z" level=warning msg="The \"SYSADMIN_EMAIL\" variable is not set. Defaulting to a blank string."
time="2026-07-07T03:33:30Z" level=warning msg="The \"DOMAIN\" variable is not set. Defaulting to a blank string."
time="2026-07-07T03:33:30Z" level=warning msg="The \"DOMAIN\" variable is not set. Defaulting to a blank string."
time="2026-07-07T03:33:30Z" level=warning msg="The \"SYSADMIN_EMAIL\" variable is not set. Defaulting to a blank string."
time="2026-07-07T03:33:30Z" level=warning msg="The \"DOMAIN\" variable is not set. Defaulting to a blank string."
time="2026-07-07T03:33:30Z" level=warning msg="The \"DOMAIN\" variable is not set. Defaulting to a blank string."
time="2026-07-07T03:33:30Z" level=warning msg="The \"SYSADMIN_EMAIL\" variable is not set. Defaulting to a blank string."
[extract-frontend-version.sh] !!!
[extract-frontend-version.sh] !!! Failed to read FRONTEND_VERSION from docker-compose.yml (got: 'null').
[extract-frontend-version.sh] !!!

And success:

How does this change impact users? Describe intentional behavior changes from code updates. What are the regression risks?

No impact.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.