Mark mojo as threadsafe

src/main/java/com/spotify/docker/PushMojo.java

@@ -36,7 +36,7 @@
 /**
  * Pushes a docker image repository to the specified docker registry.
  */
-@Mojo(name = "push")
+@Mojo(name = "push", threadSafe = true)
 public class PushMojo extends AbstractDockerMojo {
 
   /** Name of image to push. */

src/main/java/com/spotify/docker/RemoveImageMojo.java

@@ -42,7 +42,7 @@
 /**
  * Removes a docker image.
  */
-@Mojo(name = "removeImage")
+@Mojo(name = "removeImage", threadSafe = true)
 public class RemoveImageMojo extends AbstractDockerMojo {
 
   /**

src/main/java/com/spotify/docker/TagMojo.java

@@ -41,7 +41,7 @@
  * Applies a tag to a docker image. Optionally, {@code useGitCommitId} can be used to generate a
  * tag consisting of the first 7 characters of the most recent git commit ID.
  */
-@Mojo(name = "tag")
+@Mojo(name = "tag", threadSafe = true)
 public class TagMojo extends AbstractDockerMojo {
 
   /**