Clarify how docker image publishing registry is determined

Closes gh-31820

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/packaging-oci-image.adoc

@@ -373,6 +373,9 @@ The generated image can be published to a Docker registry by enabling a `publish
 If the Docker registry requires authentication, the credentials can be configured using `docker.publishRegistry` properties.
 If the Docker registry does not require authentication, the `docker.publishRegistry` configuration can be omitted.
 
+NOTE: The registry that the image will be published to is determined by the registry part of the image name (`docker.example.com` in these examples).
+If `docker.publishRegistry` credentials are configured and include a `url` property, this value is passed to the registry but is not used to determine the publishing registry location.
+
 [source,groovy,indent=0,subs="verbatim,attributes",role="primary"]
 .Groovy
 ----

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-build-image-publish.gradle

@@ -15,8 +15,6 @@ tasks.named("bootBuildImage") {
 		publishRegistry {
 			username = "user"
 			password = "secret"
-			url = "https://docker.example.com/v1/"
-			email = "[email protected]"
 		}
 	}
 }

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-build-image-publish.gradle.kts

@@ -18,8 +18,6 @@ tasks.named<BootBuildImage>("bootBuildImage") {
 		publishRegistry {
 			username = "user"
 			password = "secret"
-			url = "https://docker.example.com/v1/"
-			email = "[email protected]"
 		}
 	}
 }

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/docs/asciidoc/packaging-oci-image.adoc

@@ -343,6 +343,9 @@ The generated image can be published to a Docker registry by enabling a `publish
 If the Docker registry requires authentication, the credentials can be configured using `docker.publishRegistry` parameters.
 If the Docker registry does not require authentication, the `docker.publishRegistry` configuration can be omitted.
 
+NOTE: The registry that the image will be published to is determined by the registry part of the image name (`docker.example.com` in these examples).
+If `docker.publishRegistry` credentials are configured and include a `url` parameter, this value is passed to the registry but is not used to determine the publishing registry location.
+
 [source,xml,indent=0,subs="verbatim,attributes",tabsize=4]
 ----
 include::../maven/packaging-oci-image/docker-pom.xml[tags=docker]

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/docs/maven/packaging-oci-image/docker-pom.xml

@@ -15,8 +15,6 @@
 						<publishRegistry>
 							<username>user</username>
 							<password>secret</password>
-							<url>https://docker.example.com/v1/</url>
-							<email>[email protected]</email>
 						</publishRegistry>
 					</docker>
 				</configuration>