Splunk Logging Driver Options: Gzip, Tags, Labels, and Environment Variables

3rd chunk of `content/manuals/engine/logging/drivers/splunk.md`

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| `splunk-verify-connection`  | optional | Verify on start, that Docker can connect to Splunk server. Defaults to true.                                                                                                                                                                                                                                                               |
| `splunk-gzip`               | optional | Enable/disable gzip compression to send events to Splunk Enterprise or Splunk Cloud instance. Defaults to false.                                                                                                                                                                                                                           |
| `splunk-gzip-level`         | optional | Set compression level for gzip. Valid values are -1 (default), 0 (no compression), 1 (best speed) ... 9 (best compression). Defaults to [DefaultCompression](https://golang.org/pkg/compress/gzip/#DefaultCompression).                                                                                                                    |
| `tag`                       | optional | Specify tag for message, which interpret some markup. Default value is `{{.ID}}` (12 characters of the container ID). Refer to the [log tag option documentation](log_tags.md) for customizing the log tag format.                                                                                                                         |
| `labels`                    | optional | Comma-separated list of keys of labels, which should be included in message, if these labels are specified for container.                                                                                                                                                                                                                  |
| `labels-regex`              | optional | Similar to and compatible with `labels`. A regular expression to match logging-related labels. Used for advanced [log tag options](log_tags.md).                                                                                                                                                                                           |
| `env`                       | optional | Comma-separated list of keys of environment variables, which should be included in message, if these variables are specified for container.                                                                                                                                                                                                |
| `env-regex`                 | optional | Similar to and compatible with `env`. A regular expression to match logging-related environment variables. Used for advanced [log tag options](log_tags.md).                                                                                                                                                                               |

If there is collision between the `label` and `env` keys, the value of the `env`
takes precedence. Both options add additional fields to the attributes of a
logging message.

Below is an example of the logging options specified for the Splunk Enterprise
instance. The instance is installed locally on the same machine on which the
Docker daemon is running.

The path to the root certificate and Common Name is specified using an HTTPS
scheme. This is used for verification. The `SplunkServerDefaultCert` is
automatically generated by Splunk certificates.

Title: Splunk Logging Driver Options: Gzip, Tags, Labels, and Environment Variables

Summary

This section continues detailing configuration options for the Splunk logging driver, including settings for gzip compression (`splunk-gzip`, `splunk-gzip-level`), message tagging (`tag`), inclusion of container labels (`labels`, `labels-regex`), and environment variables (`env`, `env-regex`) in log messages. It notes that environment variables take precedence over labels if there's a conflict and provides an example of how to configure logging options for a local Splunk Enterprise instance using HTTPS and certificate verification.