Add dockerCertPath option to allow SSL connection to dockerHost

[lorellalou]

This PR allows to specify the path to the Docker Certificates created by docker-machine.

<plugin>
    <groupId>com.spotify</groupId>
    <artifactId>docker-maven-plugin</artifactId>
    <configuration>
        <dockerHost>${docker.host}</dockerHost>
        <dockerCertPath>${docker.cert.path}</dockerCertPath>
    </configuration>
</plugin>

The properties can be defined with a Maven profile in settings.xml

<profile>
    <id>docker</id>
       <properties>
           <docker.host>https://192.168.99.100:2376</docker.host
           <docker.cert.path>/Users/me/.docker/machine/machines/default</docker.cert.path>
       </properties>
</profile>

This can address #177 without ENV, only Properties in Maven profiles

[mattnworb]

it would also be fine to make the field protected and add @VisibleForTesting to it

[mattnworb]

This change LGTM, I'm just curious about the use case - docker-maven-plugin constructs the DockerClient by calling DefaultDockerClient.fromEnv() which will set the certificate path to the path pointed to by the environment variable DOCKER_CERT_PATH, if it is set.

What would be the advantage in configuring this via Maven properties or the plugin <configuration> block instead of an environment variable?

[davidxia]

That's a good point. I'd rather have one way of setting the cert path.

[lorellalou]

@mattnworb if I want to manage several docker hosts I can use docker-machine env before invoking Maven. But I think it is more appropriate to manage these docker hosts using a Maven Profile (you can't set ENV in a Maven Profile). And since you already provide the dockerHost config, I think that allowing to set also dockerCertPath is more consistent.

[mattnworb]

@lrolaz sounds reasonable to me, thanks for the explanation

[chariut]

Your solution would work only if I build locally, where I have control over my host. What would be the dockerHost and dockerCertPath values for dockerHUB? I have similar issues when I run my build in dockerHub.