Update of docker error message

What does this MR do?

This MR updates the error message displays by the docker executor when the image given is missing

Why was this MR needed?

This MR was needed to make the error entry more readable by the end user.

What's the best way to test this MR?

Run gitlab-runner with a docker executor on a project having an image other than alpine:3.11 and alpine:3.11.7

For my tests, the following configurations were used :

• .gitlab-ci.yml

image: alpine:latest

start_evaluation:
  script:
    - echo Done

• config.toml was used :

concurrent = 1
check_interval = 0

[session_server]
    session_timeout = 1800

[[runners]]
    name = "Local GitLab Runner for tests and debugging"
    url = "https://gitlab.com/"
    token = "TOKEN_HERE"
    executor = "docker"
    [runners.custom_build_dir]
    [runners.cache]
        [runners.cache.s3]
        [runners.cache.gcs]
        [runners.cache.azure]
    [runners.docker]
        tls_verify = false
        image = "alpine:3.11.7"
        privileged = false
        disable_entrypoint_overwrite = false
        oom_kill_disable = false
        disable_cache = false
        volume = ["/cache"]
        shm_size = 0
        allowed_images = ["alpine:3.11", "alpine:3.11.7"]

In the pipeline section of the project, the job should fail displaying error messages as below

Job Log file

What are the relevant issue numbers?

#26671 (closed)