Allow to use the docker build --file option when building an image

We would like to NOT have the Dockerfile at the root of the project, and have everything docker related in a subfolder "docker".

Then we would build the image with
docker build -t myrepo/myproject:v01 --file docker/Dockerfile .
'.' is the docker context, meaning even though the Dockerfile is in a subfolder 'docker', docker can still access everything under '.' (the root of the project).
It is not possible otherwise to access folders outside of the context, in a Dockerfile (using
ADD../src/myfile.jar /srv/lib/myfile.jar
,for example. '..' is not allowed).

So it would be nice to still have the options, in the GUI:

• "Directory Dockerfile is in" (equivalent of "--file <path>)
• "Docker context".

Note:
In the code, this seems to be in function buildAndTag() in file:

src/main/java/com/cloudbees/dockerpublish/DockerBuilder.java

{{String context = defined(getDockerfilePath()) ?
getDockerfilePath() : ".";

lastResult = executeCmd("docker build -t " + i.next()
+ ((isNoCache()) ? " --no-cache=true " : "") + " "
+ ((isForcePull()) ? " --pull=true " : "") + " "
+ context);}}

The line
{{String context = defined(getDockerfilePath()) }}
should be changed so that DockerfilePath is not the same as context. Those should be different things.