Splunkbase app not installed when using SPLUNK_APPS_URL

If the compose.yml file contains a reference to a SPLUNKBASE app URL like:

  so1:
    environment:
      SPLUNK_APPS_URL: https://splunkbase.splunk.com/app/1924/release/7.2.1/download
      SPLUNK_PASSWORD: <password>
      SPLUNK_STANDALONE_URL: so1
      SPLUNK_START_ARGS: --accept-license
      SPLUNKBASE_PASSWORD: <sb_password>
      SPLUNKBASE_USERNAME: <sb_user>
      TZ: Europe/Brussels

the container fails to start with the following error message:

so1-1  | TASK [splunk_standalone : Install app via REST] ********************************
so1-1  | fatal: [localhost]: FAILED! => {}
so1-1  | 
so1-1  | MSG:
so1-1  | 
so1-1  | The conditional check ''itsi' not in app_contents.stdout_lines' failed. The error was: error while evaluating conditional ('itsi' not in app_contents.stdout_lines): 'dict object' has no attribute 'stdout_lines'
so1-1  | 
so1-1  | The error appears to be in '/opt/ansible/roles/splunk_common/tasks/install_apps.yml': line 73, column 3, but may
so1-1  | be elsewhere in the file depending on the exact syntax problem.
so1-1  | 
so1-1  | The offending line appears to be:
so1-1  | 
so1-1  | 
so1-1  | - name: Install app via REST
so1-1  |   ^ here
so1-1  | 
so1-1  | 
so1-1  | PLAY RECAP *********************************************************************
so1-1  | localhost                  : ok=82   changed=17   unreachable=0    failed=1    skipped=84   rescued=0    ignored=0  

Reproduce the failure with the SPLUNK_APPS_URL example in compose.yml, then inspect the Install app via REST task at /opt/ansible/roles/splunk_common/tasks/install_apps.yml line 73. Confirm the fix by starting the container successfully and verifying that the Splunkbase app is installed.