List process container images in preview mode

modules/nextflow/src/main/groovy/nextflow/Session.groovy

@@ -468,13 +468,27 @@ class Session implements ISession {
         CH.broadcast()
 
         if( preview ) {
+            dumpContainerImages()
             terminated = true
         }
         else {
             callIgniters()
         }
     }
 
+    private void dumpContainerImages() {
+        for( def vertex : dag.vertices ) {
+            // skip nodes that are not processes
+            if( !vertex.process )
+                continue
+
+            // print process name and container image
+            def process = vertex.process
+            log.debug "Querying container image for process `${process.name}`..."
+            log.info "${process.name} ${process.getContainer()}"
+        }
+    }
+
     private void callIgniters() {
         log.debug "Igniting dataflow network (${igniters.size()})"
         for( Closure action : igniters ) {

modules/nextflow/src/main/groovy/nextflow/processor/TaskProcessor.groovy

@@ -367,6 +367,23 @@ class TaskProcessor {
 
     boolean hasErrors() { errorCount>0 }
 
+    /**
+     * Get the container image of the process, or return
+     * null if it can't be resolved.
+     */
+    String getContainer() {
+        try {
+            def config = config.createTaskConfig()
+            config.context = new TaskContext(this)
+
+            return config.getContainer()
+        }
+        catch( Exception e ) {
+            log.warn1 "Unable to resolve container image for process `${name}`", e
+            return null
+        }
+    }
+
     protected void checkWarn(String msg, Map opts=null) {
         if( NF.isStrictMode() )
             throw new ProcessUnrecoverableException(msg)