[fix] Specify the dockerfile path when building under windows, the re…

plugin/src/main/java/com/spotify/plugin/dockerfile/BuildMojo.java

@@ -38,6 +38,8 @@
 import java.util.regex.Pattern;
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+
+import org.apache.commons.io.FilenameUtils;
 import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.plugin.MojoFailureException;
 import org.apache.maven.plugin.logging.Log;
@@ -70,6 +72,7 @@ public class BuildMojo extends AbstractDockerMojo {
   @Parameter(property = "dockerfile.dockerfile", required = false)
   private File dockerfile;
 
+
   /**
    * The repository to put the built image into when building the Dockerfile, for example
    * <tt>spotify/foo</tt>.  You should also set the <tt>tag</tt> parameter, otherwise the tag
@@ -179,8 +182,8 @@ static String buildImage(@Nonnull DockerClient dockerClient,
 
     final ArrayList<DockerClient.BuildParam> buildParameters = new ArrayList<>();
     if (dockerfile != null) {
-      buildParameters.add(DockerClient.BuildParam.dockerfile(
-          contextDirectory.relativize(dockerfile)));
+      buildParameters.add(new DockerClient.BuildParam("dockerfile",
+              FilenameUtils.separatorsToUnix(contextDirectory.relativize(dockerfile).toString())));
     }
 
     final LoggingProgressHandler progressHandler = new LoggingProgressHandler(log, verbose);