I am not sure if you can specify Dockerfile, but you can use cloudbuild.yaml file. Check gcloud documentation. If you want to rename this file, you can use config option.

    gcloud builds submit --config cloudbuild.yaml .

A sample cloudbuild.yaml file look like this,

steps:
- name: 'gcr.io/cloud-builders/docker'
  args: [ 'build', '-t', 'gcr.io/$PROJECT_ID/quickstart-image', '.' ]
images:
- 'gcr.io/$PROJECT_ID/quickstart-image'

The only way to specify a Dockerfile (i.e. other than ./Dockerfile) would be to create a cloudbuild.yaml per techtabu@. This config could then use the docker builder and provide the specific Dockerfile, i.e.:

steps:
- name: "gcr.io/cloud-builders/docker"
  args:
  - build
  - "--tag=gcr.io/$PROJECT_ID/quickstart-image"
  - "--file=./path/to/YourDockerFile"
  - .
...
images:
- "gcr.io/$PROJECT_ID/quickstart-image"

If you wish, you also get to specify an alternative name than cloudbuild.yaml.

The ./Dockerfile assumption is presumably to ease the transition to Cloud Build.

I recommend you switch to using cloudbuild.yaml for the flexibility it provides.

You can very easily do this by substituting the . by ./path/to/YourDockerFile, so the gcloud command will be:

gcloud builds submit --tag gcr.io/[PROJECT_ID]/quickstart-image ./path/to/YourDockerFile

So you don't have to use a cloudbuild.yaml for this.