Accept image names with library/ prefix as a valid substitute

[eddumelendez]

Docker official images can be pulled with or without library/
prefix as part of the image name. For example, mysql or
library/mysql. This is explicit when using private repositories.

Container classes such as CassandraContainer, for example, it
already considers as a valid substitution cassandra.
This change, by default adds a new substitution with library/
prefix.

[bsideup]

@eddumelendez have you considered having it in assertCompatibleWith instead? Would especially be helpful if the prefix will change to something else one day (since it is implicit in Docker) 👍

[kiview]

Thanks a lot for providing the context here in the PR @eddumelendez and thanks to @bsideup for more ideas.

My preferred way initially was to combine both changes (either within this PR, or 2 distinct PRs):

1. Have assertCompatibleWith be aware of the library prefix for all images (indeed, we don't need to handle it for specific images)
2. Internally specify images using library/ prefix, to solve issues such as [Enhancement]: Prepend "library/" to official docker image "alpine" #6166 or [Bug]: library/mysql is not a valid substitute for mysql #5612 in a very transparent way.

Reflecting on this, I see no drawback with doing 1), especially since it is a very local change. Regarding 2) I am now unsure. What binds is more strongly to Docker implementation details and is more prone to breaking, using mysql or library/mysql? I think it is still fine to integrate both changes, WDYT?

[bsideup]

Internally specify images using library/ prefix

I am not sure about this one. This indeed exposes Docker's internal impl and, should they change it, we might be in trouble. I see no reason to touch how we define the images (redis:6 is an absolutely valid Docker image name, but I do see how treating library/-prefixed image names makes sense, because these are indeed compatible. So, can we please proceed with just 1?

[kiview]

@bsideup Yes makes sense, we will proceed with 1) only.

[kiview]

I think the implementation becomes much more straightforward if moved to isCompatibleWith(). Something along the lines of:

     public boolean isCompatibleWith(DockerImageName other) {
         // is this image already the same or equivalent?
-        if (other.equals(this)) {
+        DockerImageName imageWithLibraryPrefix = DockerImageName.parse("library/" + other.repository);
+        if (other.equals(this) || imageWithLibraryPrefix.equals(this)) {
             return true;
         }
        [...]
     }

[driverpt]

(Minor) More cleaner

if (Objects.equals(other, this) || Objects.equals(imageWithLibraryPrefix, this)) {
   ....
}

[kiview]

I don't think this changes the degree of "cleanness", but a question of taste (beside the null-safety of Objects.equals() in general). Either style should be fine in this case.

[kiview]

LGTM, I much prefer the implementation in isCompatibleWith() 👍
See the open comment, I think we can merge once adressed.

[kiview]

I don't think this changes the degree of "cleanness", but a question of taste (beside the null-safety of Objects.equals() in general). Either style should be fine in this case.

[kiview]

The comment makes no more sense here like this. Instead, we can add a comment saying:

// Make sure we always compare against a version of the image name containing the LIBRARY_PREFIX