Gitlab job is stuck, no active runners, why?

Question

I have a step that builds a docker image. Normally, I'd have this done when a merge happens, but in this case, I want to prove it works, so I'm trying to force it to run when as part of the normal build. Yet, regardless Gitlab (the cloud version) cannot or will not find a "runner" to get this done. This is my .gitlab-ci.yml file. The stage/step in question is

build-docker-image

This job stays in pending with the following message:

This job is stuck because of one of the following problems. There are no active runners online, no runners for the protected branch , or no runners that match all of the job's tags: build-docker-image
Go to project CI settings

stages:
  - build-app
  - test
  - build-docker-image
  - deploy-develop
  - deploy-staging
  - deploy-prod

variables:
  APP: myapp

##########################################
# Build
##########################################
build-app:
  stage: build-app
  image:
    name: mirror.gcr.io/library/maven:3.8.4-openjdk-17
  variables:
    GIT_DEPTH: 5
  cache:
    key: Maven
    paths:
      - .mvn/
  artifacts:
    paths:
      - target/myapp-*.jar
    expire_in: 1 month
  tags:
    - build
  script:
    - mvn -DskipTests -Dmaven.repo.local=$(pwd)/.mvn package

##########################################
# Test
##########################################
test:
  stage: test
  image:
    name: mirror.gcr.io/library/maven:3.8.4-openjdk-17
  variables:
    GIT_DEPTH: 51
  cache:
    key: Maven
    paths:
      - .mvn/
  artifacts:
    paths:
      - target/myapp-*.jar
    expire_in: 1 month
  tags:
    - build
  script:
    - mvn test -Dmaven.repo.local=$(pwd)/.mvn test

##########################################
# Build Docker Image
##########################################
build-docker-image:
  stage: build-docker-image
  variables:
    GIT_DEPTH: 5
  image:
    name: gcr.io/kaniko-project/executor:debug
    entrypoint: [""]
  dependencies:
    - build-app
  tags:
    - build-docker-image
  script:
    - mkdir -p /kaniko/.docker
    - echo $GCP_DOCKER | base64 -d > /kaniko/.docker/config.json
    - >-
      /kaniko/executor
      --context "${CI_PROJECT_DIR}"
      --dockerfile "${CI_PROJECT_DIR}/Dockerfile"
      --destination "us.gcr.io/mycompany-ops/${CI_PROJECT_NAME}:${CI_COMMIT_SHORT_SHA}"
      --build-arg GIT_SHA=${CI_COMMIT_SHORT_SHA}
      --build-arg GIT_TAG=${CI_COMMIT_TAG}
      --build-arg BUILT_ON="$(date)"
  when: 
#  only:
#    - main

Answer 1

Tags within .gitlab-ci.yml map jobs to runners. If you have no runners tagged with build-docker-image, then you will get the following error message:

This job is stuck because of one of the following problems. There are no
active runners online, no runners for the protected branch, or no runners
that match all of the job's tags: build-docker-image

If you want to use the same runner as other steps in your pipeline, change the following:

##########################################
# Build Docker Image
##########################################
build-docker-image:
  stage: build-docker-image
  # ...
  tags:
    - build
  script:
  # ...