CI: Update GitHub Action to build Docker container

[xnyo]

Updates the GitHub Action test.yml so it builds the Docker container, rather than running the tests and build step directly inside GitHub Actions.

Right now, the GitHub Action is doing the same steps as the ones in the Dockerfile:

plugin-validator/Dockerfile

Lines 1 to 16 in 45b483c

	FROM golang:1.21-alpine as builder
	WORKDIR /go/src/github.com/grafana/plugin-validator
	ADD . /go/src/github.com/grafana/plugin-validator
	RUN apk add --no-cache git ca-certificates curl && \
	update-ca-certificates
	RUN git clone https://github.com/magefile/mage --depth 1 && \
	cd mage && \
	go run bootstrap.go && \
	curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(go env GOPATH)/bin v1.55.2
	RUN cd /go/src/github.com/grafana/plugin-validator && \
	mage -v build:ci && \
	ls -al bin

With this PR, the GH action will trigger a Docker image build. This is better because whenever we change the build/test steps (or the golangci-lint, semgrep or gosec versions), we don't have to update the GH action workflow as well as the Dockerfile.

This is also the same approach used in the drone pipeline, where the "build + test + push" step simply builds the Docker container:

plugin-validator/.drone.yml

Lines 4 to 14 in 45b483c

	steps:
	- image: plugins/docker
	name: build + test + push
	settings:
	dry_run: true
	password:
	from_secret: docker-hub-password
	repo: grafana/plugin-validator-cli
	tags: latest
	username:
	from_secret: docker-hub-username