patch custom image

[kennyhyun]

in provider.docker,

• add registry like registry.gitlab.com
• add imagePullSecrets like [{ name: 'secret-name' }]

in functions.x

• add image like kennyhyun/container
• add tagPrefix like knative-hello- for custom tag

for using registry.gitlab.com/kennyhyun/container:knative-hello-123456789 as a image url

[kennyhyun]

Sorry, I've accidentally chosen wrong target branch to merge. wanted to merge into my fork