Tidy Gradle dockerBuildX target

Build linux/amd64, linux/arm64, and linux/arm/v7 (arm 32-bit) targets
all at once.
This still requires Gradle and JDK on the local machine (i.e. outside of
the Docker build container), because it seems pointless to build the
.jar 3 times, once for each architecture, but tags all the images with
the same tag, rather than using a tag for each architecture. This allows
clients to all use the same image tag, but pull down the architecture
that's right for them.

I'd like to have it build the .jar file in a container (so the host
doesn't need JDK and Gradle), but couldn't think how to do that
efficiently (i.e. only once), with this approach.

build.gradle

@@ -275,22 +275,10 @@ task dockerBuild(type: Exec) {
 }
 
 task dockerBuildX(type: Exec, dependsOn: shadowJar) {
-    commandLine 'docker', 'buildx', 'build', '--load', '--platform', 'linux/arm64', '-t', dockerImageTag + '-arm64', project.rootDir
+    commandLine 'docker', 'buildx', 'build', '--push', '--platform', 'linux/arm64,linux/amd64,linux/arm/v7', '-t', dockerImageTag , project.rootDir
     doLast {
         exec {
-            commandLine 'docker', 'buildx', 'build', '--load', '--platform', 'linux/amd64', '-t', dockerImageTag + '-amd64', project.rootDir
-        }
-
-        exec {
-            commandLine 'docker', 'tag', dockerImageTag + '-arm64', "${dockerImageName}:latest-arm64-dev"
-        }
-
-        exec {
-            commandLine 'docker', 'tag', dockerImageTag + '-amd64', "${dockerImageName}:latest-amd64-dev"
-        }
-
-        exec {
-            commandLine 'docker', 'tag', dockerImageTag + '-amd64', "${dockerImageName}:latest-dev"
+            commandLine 'docker', 'buildx', 'build', '--push', '--platform', 'linux/arm64,linux/amd64,linux/arm/v7', '-t', "${dockerImageName}:latest-dev", project.rootDir
         }
     }
 }