Take a look at https://quarkus.io/guides/container-image#quarkus-container-image_quarkus.container-image.group this should answer your question(s).

He Anand, auch hier die Frage, hat dir die Doku weitergeholfen?

So here fundamentally I have to use the quarkus.container-image.name in case I want to override the image name that you are providing by default. The other properties, like I said in the question before if i only change quarkus.container-image.registry-name, it is taking my username as the default, as the document says.
So I have to kind of hardcode the quarkus.container-image.name to include registry name, image name and tag name.
On a slightly different note, I still don't have a CI pipeline to build. If you have a sample gitlab CI Pipeline for me, that just runs
mvn clean install conx:package could you please share it.

for native image build

stages:
  - build
  - notify

variables:
  MAVEN_OPTS: "-Dmaven.repo.local=$CI_PROJECT_DIR/.m2/repository -Dorg.slf4j.simpleLogger.showDateTime=true -Djava.awt.headless=true"
  MAVEN_CLI_OPTS: "--batch-mode --errors --fail-at-end --no-transfer-progress --show-version -DdeployAtEnd=true"
  QUARKUS_JIB_BASE_REGISTRY_USERNAME: $DOCKER_USER
  QUARKUS_JIB_BASE_REGISTRY_PASSWORD: $DOCKER_PASSWORD
  QUARKUS_CONTAINER_IMAGE_USERNAME: $DOCKER_USER
  QUARKUS_CONTAINER_IMAGE_PASSWORD: $DOCKER_PASSWORD

cache:
  paths:
    - .m2/repository/

build:snapshot:
  image: dockr.swgt.at/quarkus/ubi-quarkus-native-image:21.3.2-java17
  stage: build
  variables:
    VERSION: $CI_COMMIT_SHORT_SHA
  only:
    - main
  tags:
    - ...
  script:
    #- ./mvnw versions:set -DnewVersion=$VERSION
    - ./mvnw $MAVEN_CLI_OPTS -Dbuild.revision=$VERSION prepare-package -DskipTests=true
    - ./mvnw $MAVEN_CLI_OPTS -Dbuild.revision=$VERSION at.campusonline.conx.maven:conx-maven-plugin:package package -DskipTests=true -P native
  artifacts:
    paths:
      - target/*.zip

build:production:
  image: dockr.swgt.at/quarkus/ubi-quarkus-native-image:21.3.2-java17
  stage: build
  variables:
    VERSION: $CI_COMMIT_REF_NAME
    COMMIT_ID: $CI_COMMIT_SHORT_SHA
  only:
    - /^(\d+\.)(\d+\.)(\d+)(-[a-z]+)?$/
  except:
    - branches
  tags:
    - ...
  script:
    - ./mvnw versions:set -DnewVersion=$VERSION
    - ./mvnw $MAVEN_CLI_OPTS -Dbuild.revision=$COMMIT_ID prepare-package -DskipTests=true
    - ./mvnw $MAVEN_CLI_OPTS -Dbuild.revision=$COMMIT_ID at.campusonline.conx.maven:conx-maven-plugin:package package -DskipTests=true -P native
  artifacts:
    paths:
      - target/*.zip
  when: manual