feat: expose current container to templates

[JamBalaya56562]

Problem

When docker-gen runs standalone (not bundled with nginx-proxy) with -only-exposed / -only-published, the docker-gen container itself exposes no ports and is dropped by the server-side container filter. Templates that resolve the current container via where $ "ID" .Docker.CurrentContainerID | first then get nil, even though .Docker.CurrentContainerID is detected correctly.

Change

• Adds a CurrentContainer() method on the root Context, returning the *RuntimeContainer whose ID matches .Docker.CurrentContainerID (or nil), so templates can write {{ with .CurrentContainer }}…{{ end }}.
• In getContainers, after the filtered list is built, the current container is inspected and appended when its ID is non-empty and not already present — so it is available even when -only-exposed/-only-published would otherwise filter it out.

Implementation notes

• The per-container inspect→RuntimeContainer conversion is extracted into an inspectContainer helper so the same logic serves both the list loop and the always-include path, without duplication.
• The current-container lookup goes through a small package-level indirection (getCurrentContainerID) so the append path can be unit-tested without a live /proc.

Tests

• TestCurrentContainer covers the match / no-match / empty-ID cases of the accessor.
• TestGetContainersIncludesCurrentContainer verifies the container is appended when it is absent from the filtered list.
• README documents .CurrentContainer.

Fixes #458