fix(docker): merge YAML objects into nested proxy labels (#219)

Sort proxy.* keys by dot depth, then name, before building the tree so
broader paths apply before deeper ones. When a new value would sit on a
node that is already a map, parse it as a YAML object (tabs normalized to
two spaces), deep-merge, and treat an empty string as an empty object.
Return clear errors when a scalar and a nested map disagree.

Drop the preallocated refPrefixes table in favor of refPrefix(n). Add
internal tests for parseLabelObject, mergeLabelMaps, key order, and
flatten; extend export tests for mixed OIDC-style labels and conflicts.

* refactor(docker): extract label parse and flatten helpers

Refactor ParseLabels by moving proxy label application into applyLabel,
descendLabelMap, and setLabelValue so traversal and leaf merge share one path
without labelLoop continues.

Add splitAliasLabel for ExpandWildcard so proxy.* prefix handling stays in one
place and uses CutPrefix/Cut consistently.

Deduplicate flattenMap and flattenMapAny value handling with flattenValue plus
joinLabelKey and stringifyLabelKey for flattened key construction.

* refactor(docker): structured errors for label type clashes

Replace ad hoc fmt.Errorf messages in descendLabelMap, setLabelValue, and
mergeLabelMaps with UnexpectedTypeError so wording is consistent and mapping
vs scalar conflicts stay explicit.

Hoist requireMap in label tests to a shared helper.

Normalize tabs to two spaces in expandYamlWildcard so wildcard YAML matches
the indentation used in the object-merge path.

* refactor(docker): optional UnexpectedTypeError message for merge conflicts

Extend UnexpectedTypeError with an optional Message field; when set, Error()
returns it instead of the default expect-versus-actual formatting.

mergeLabelMaps sets that message when a mapping would merge into an existing
scalar, so the error states the situation instead of only "expect scalar".

Update TestMergeLabelMaps to assert the new wording.

config.example.yml

@@ -52,8 +52,8 @@ entrypoint:
   # Note that HTTP/3 with proxy protocol is not supported yet.
   support_proxy_protocol: false
 
-  # To relay the client address to a TCP upstream, enable `relay_proxy_protocol_header: true`
-  # on that specific TCP route. UDP relay is not supported yet.
+  # To relay the client address to a TCP upstream (UDP relay is not supported yet)
+  relay_proxy_protocol_header: false
 
   # Below define an example of middleware config
   # 1. set security headers