Description of problem:

As a user, dockerfile pipeline flow should allow me to customize the path to the Dockerfile in a git repository.

openshift pipelines 1.2 operator has a new param called PATH_CONTEXT (in 4.7 used as DOCKERFILE param in this #PR). We need to set this param to allow the user to have custom path to the dockerfile.

This needs to be backported to 4.6 as well, as 1.2 operator will be used in the ocp-4.6 channel.

Prerequisites (if any, like setup, operators/versions):

Openshift pipelines operator 1.2

Steps to Reproduce

1. In Add page, choose the dockerFile flow
2. Add a git repository with dockerfile located in custom path
   https://github.com/karthikjeeyar/node-docker-good-defaults
3. add custom path in the DOCKERFILE form field  

Actual results: 

Pipeline execution fails with `no such file or directory` in build task.

Expected results: 

Pipeline execution should pass when the custom path is provided in the form.

Reproducibility (Always/Intermittent/Only Once): 

Always

Build Details:

OCP - 4.7 & 4.6

Additional info: 

Similar ticket that supported other params like GIT_REPO, GIT_REVISION, IMAGE_NAME is here https://issues.redhat.com/browse/ODC-4466

buildah task : https://raw.githubusercontent.com/openshift/tektoncd-pipeline-operator/master/pkg/utils/addons/testdata/buildah.golden