[ci] more precise regex for renovate images detection

[jkroepke]

What this PR does / why we need it

this should fix wrong image detection on prometheus helm chart.

Old regex: https://regex101.com/r/VileVj/1

Ref:

helm-charts/charts/prometheus/values.yaml

Lines 160 to 179 in ae8ccb2

	# sidecarContainers - add more containers to prometheus server
	# Key/Value where Key is the sidecar `- name: <Key>`
	# Example:
	# sidecarContainers:
	# webserver:
	# image: nginx
	# OR for adding OAuth authentication to Prometheus
	# sidecarContainers:
	# oauth-proxy:
	# image: quay.io/oauth2-proxy/oauth2-proxy:v7.1.2
	# args:
	# - --upstream=http://127.0.0.1:9090
	# - --http-address=0.0.0.0:8081
	# - ...
	# ports:
	# - containerPort: 8081
	# name: oauth-proxy
	# protocol: TCP
	# resources: {}
	sidecarContainers: {}

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #

Special notes for your reviewer

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])

[GMartinez-Sisti]

DCO missing 😝