Add default dockerfile path to skaffold config when using skaffold init

[MarlonGamez]

Fixes #3766

Description
This PR is meant to address users who run skaffold init and are given the wrong context field in their skaffold config for docker artifacts. Currently the analyzer simply sets the context as the dockerfile location, but this may be different from the actual root of the project's source code.

Given a project structure like so:

proj-dir
├── README.md
└── app
    ├── deploy
    │   ├── Dockerfile
    │   └── k8s-pod.yaml
    └── main.go

Before, a user running skaffold init would get something like this:

apiVersion: skaffold/v2beta9
kind: Config
metadata:
  name: getting-started
build:
  artifacts:
  - image: skaffold-example
    context: app/deploy
deploy:
  ...

Now, they will get something like

apiVersion: skaffold/v2beta9
kind: Config
metadata:
  name: getting-started
build:
  artifacts:
  - image: skaffold-example
    context: app/deploy
    docker:
      dockerfile: Dockerfile
deploy:
  ...

This will allow users to more easily alter the necessary fields of their config to work for their project, as mentioned in #3766

Follow-up Work
The issue at the root of this is really #3760. I'd like to list this as follow up work as tackling this issue will create a much better skaffold init experience by reducing the need for modification of the config after the fact.

[codecov]

Codecov Report

Merging #4989 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4989      +/-   ##
==========================================
- Coverage   72.25%   72.19%   -0.07%     
==========================================
  Files         361      362       +1     
  Lines       12704    12715      +11     
==========================================
  Hits         9179     9179              
- Misses       2843     2855      +12     
+ Partials      682      681       -1     

Impacted Files	Coverage Δ
pkg/skaffold/docker/docker_init.go	100.00% <ø> (ø)
pkg/skaffold/build/local/docker.go
pkg/skaffold/build/docker/docker.go	42.00% <0.00%> (ø)
pkg/skaffold/build/docker/types.go	100.00% <0.00%> (ø)
pkg/skaffold/build/local/types.go	78.84% <0.00%> (+1.92%)	⬆️
pkg/skaffold/build/local/local.go	72.50% <0.00%> (+12.08%)	⬆️

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