serverId overrides default docker-hub repo

[maven2016]

Description

[Add feature/bug description here]
Bug - serverId overrides default docker-hub repo

How to reproduce

I'm using Dockerfile:
FROM java:8

pom.xml has configured which points to maven-settings-xml with the private repo details.

What do you expect

i'd expect the Dockerfile be able to pull the image from the default public repo instead of accessing the private repo.

What happened instead

I get the following error:
Step 0 : FROM java:8
Pulling repository java
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 45.166s
[INFO] Finished at: Sat Aug 06 11:17:17 UTC 2016
[INFO] Final Memory: 34M/595M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal com.spotify:docker-maven-plugin:0.4.11:build (default-cli) on project server: Exception caught: Authentication is required. -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal com.spotify:docker-maven-plugin:0.4.11:build (default-cli) on project server: Exception caught
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:217)

[Describe the actual results]

The error received says "authentication is required".
two solutions:
if I remove from the pom file, i don't get the error above - however i can't push the image to my private repo.

if i pull java:8 manually beforehand, i can add to the pom file and i don't receive the error above.

Software:

• docker version: [Add docker version here, client and server]
  client:1.6.2
  server:1.6.2
• docker-maven-plugin version: [Add docker-maven-plugin version here]
  0.4.11
• maven version: [Add maven version here]
  maven:3.0.5

Full backtrace

[Paste full backtrace here]

[s17t]

This is a duplicate of later #140

[davidxia]

I am not sure if this is still an issue, since the reports are pretty old, but #339 fixes all of the root causes.