feat(Trivy): allow Trivy to be run in standalone mode

This job performs a Vulnerability Static Analysis with [Trivy](https://github.com/aquasecurity/trivy) on your built image.

:warning: As presented below, this job is enabled only if you specify a Trivy server address with the `DOCKER_TRIVY_ADDR` environment variable.

Without any configuration Trivy will run in [standalone](https://aquasecurity.github.io/trivy/v0.28.0/docs/references/modes/standalone/) mode.

A Trivy server has been deployed internally. If you want to use it, you can add the following variable definition to you `.gitlab-ci.yml`:

If you want to run Trivy in client/server mode, you need to set the `DOCKER_TRIVY_ADDR` environment variable.

```yaml

variables:

| Name                   | Description                            | Default value     |

| ---------------------- | -------------------------------------- | ----------------- |

| `DOCKER_TRIVY_IMAGE`   | The docker image used to scan images with Trivy | `aquasec/trivy:latest` |

| `DOCKER_TRIVY_ADDR`    | The Trivy server address               | _(none: disabled by default)_  |

| `DOCKER_TRIVY_ADDR`    | The Trivy server address (for client/server mode)              | _(none: standalone mode)_  |

| `DOCKER_TRIVY_SECURITY_LEVEL_THRESHOLD`| Severities of vulnerabilities to be displayed (comma separated values: `UNKNOWN`, `LOW`, `MEDIUM`, `HIGH`, `CRITICAL`) | `UNKNOWN,LOW,MEDIUM,HIGH,CRITICAL`  |

| `DOCKER_TRIVY_DISABLED`| Set to `true` to disable Trivy analysis          | _(none)_ |

| `DOCKER_TRIVY_ARGS`    | Additional [`trivy client` arguments](https://aquasecurity.github.io/trivy/v0.27.1/docs/references/cli/client/)  | `--ignore-unfixed --vuln-type os` |

    name: $DOCKER_TRIVY_IMAGE

    entrypoint: [""]

  stage: package-test

  variables:

    TRIVY_CACHE_DIR: ".trivycache/"

  script: |

    # cache cleanup is needed when scanning images with the same tags, it does not remove the database

    trivy image --clear-cache

    export TRIVY_USERNAME=${DOCKER_REGISTRY_SNAPSHOT_USER:-${DOCKER_REGISTRY_USER:-$CI_REGISTRY_USER}}

    export TRIVY_PASSWORD=${DOCKER_REGISTRY_SNAPSHOT_PASSWORD:-${DOCKER_REGISTRY_PASSWORD:-$CI_REGISTRY_PASSWORD}}

    export FILENAME=$(echo "${DOCKER_SNAPSHOT_IMAGE}" | sed 's|[/:]|_|g')

    mkdir -p ./trivy

    if [[ -z "${DOCKER_TRIVY_ADDR}" ]]; then

      log_warn "\\e[93mYou are using Trivy in standalone mode. To get faster scans, consider setting the DOCKER_TRIVY_ADDR variable to the address of a Trivy server. More info here: https://aquasecurity.github.io/trivy/latest/docs/references/modes/client-server/\\e[0m"

      trivy image --download-db-only

      export trivy_opts="image"

    else

      log_info "You are using Trivy in client/server mode with the following server: ${DOCKER_TRIVY_ADDR}"

      export trivy_opts="image --server ${DOCKER_TRIVY_ADDR}"

    fi

    # Add common trivy arguments

    export trivy_opts="${trivy_opts} --severity ${DOCKER_TRIVY_SECURITY_LEVEL_THRESHOLD} --vuln-type os ${DOCKER_TRIVY_ARGS}"

    # the first execution of Trivy should never fail, otherwise the other executions won't be run (so --exit-code=0)

    trivy client --remote ${DOCKER_TRIVY_ADDR} --format template --template @/contrib/junit.tpl --severity "${DOCKER_TRIVY_SECURITY_LEVEL_THRESHOLD}" --output ./trivy/${FILENAME}.xml --exit-code 0  ${DOCKER_TRIVY_ARGS} $DOCKER_SNAPSHOT_IMAGE

    trivy client --remote ${DOCKER_TRIVY_ADDR} --format json --severity "${DOCKER_TRIVY_SECURITY_LEVEL_THRESHOLD}" --output ./trivy/${FILENAME}.json --exit-code 0 ${DOCKER_TRIVY_ARGS} $DOCKER_SNAPSHOT_IMAGE

    trivy client --remote ${DOCKER_TRIVY_ADDR} --format table --severity "${DOCKER_TRIVY_SECURITY_LEVEL_THRESHOLD}" --exit-code 1 ${DOCKER_TRIVY_ARGS} $DOCKER_SNAPSHOT_IMAGE

    trivy ${trivy_opts} --format template --template @/contrib/junit.tpl --output ./trivy/${FILENAME}.xml --exit-code 0  $DOCKER_SNAPSHOT_IMAGE

    trivy ${trivy_opts} --format json --output ./trivy/${FILENAME}.json --exit-code 0 $DOCKER_SNAPSHOT_IMAGE

    trivy ${trivy_opts} --format table --exit-code 1 $DOCKER_SNAPSHOT_IMAGE

  artifacts:

    when: always

    paths:

    - trivy/

    reports:

      junit: "trivy/*.xml"

  cache:

    paths:

      - .trivycache/

  rules:

    - if: '$DOCKER_TRIVY_DISABLED == "true"'

      when: never

    - if: '$DOCKER_TRIVY_ADDR && ($CI_COMMIT_REF_NAME =~ $PROD_REF || $CI_COMMIT_REF_NAME =~ $INTEG_REF)'

    - if: '($CI_COMMIT_REF_NAME =~ $PROD_REF || $CI_COMMIT_REF_NAME =~ $INTEG_REF)'

    # allow failure on development branches:

    - if: $DOCKER_TRIVY_ADDR

      allow_failure: true

    - allow_failure: true

# ==================================================