[docker] Declare docker in the services tuple

[suraj-cmd]

setup() calls add_journal() for the docker unit but the plugin declares no
services tuple and never collects the service status. An sosreport from a
host where dockerd has failed to start therefore contains the journal but
nothing showing whether the unit is loaded, enabled or running.

Plugin._collect_services() runs each entry of the tuple through
is_service() and calls both add_service_status() and add_journal(), so
declaring the unit adds the missing status and replaces the explicit call.

It also gives the plugin an enablement trigger beyond the package name. The
base class declares no packages and the two distribution subclasses list ten
names between them, while the unit is docker in every case.

The SoSPredicate on the following line is unaffected — it gates whether the
docker subcommands run, which is separate from the plugin's services
attribute.

Follow-up to #4443, as offered there.

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[packit-as-a-service]

Congratulations! One of the builds has completed. 🍾

You can install the built RPMs by following these steps:

• sudo dnf install -y 'dnf*-command(copr)'
• dnf copr enable packit/sosreport-sos-4444
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.