[system testing] service_notify_signal fails when the service references env vars

[andrewkroh]

If a system test uses service_notify_signal: SIGHUP and the _dev/deploy/docker/docker-compose.yml file references environment variables like ${SERVICE_LOGS_DIR}, then the signalling can fail. This must be new validation in docker, because this used to work.

My machine is using:

Docker Engine: 20.10.22
Docker Compose: v2.15.1
Docker for Mac: 4.16.1 (95567)

2023/01/25 20:12:21 DEBUG running command: /usr/local/bin/docker-compose -f /Users/akroh/code/elastic/integrations/packages/cef/_dev/deploy/docker/docker-compose.yml -p elastic-package-service kill -s SIGHUP cef-log-udp
WARN[0000] The "SERVICE_LOGS_DIR" variable is not set. Defaulting to a blank string. 
1 error(s) decoding:

* error decoding 'Volumes[1]': invalid spec: :/var/log: empty section between colons
2023/01/25 20:12:21 DEBUG reassigning original policy back to agent...

Error: error running package system tests: could not complete test run: failed to notify test service: could not send "SIGHUP" signal: running Docker Compose kill command failed: exit status 15

[andrewkroh]

I think that when I last updated Docker for Mac that my settings got lost because compose v2 was re-enabled. I had it disabled.

[andrewkroh]

In any case, the fix in #1115 corrects the issue.