Allow default image customization without tags

I would like the ability to customize the default images used by testcontainers but NOT specify the specific tags used. Therefore, allowing the default tags to continue to be specified by testcontainers.

In my corporate environment, we have an artifactory instance that caches several public repositories (docker.io, quay.io, etc). Our builds generally pull from these caches instead of the public repositories directly.

I want to tell testcontainers to use our caches instead of the public repositories, but I don't want to tell testcontainers which specific tag I want to use. I want to rely on the default tags specified in testcontainers, so I don't have to constantly keep updating the tags used whenever I upgrade testcontainers of various builds.

Currently the default images used by testcontainers can be customized by specifying properties like this:

tinyimage.container.image = alpine:3.5
vncrecorder.container.image = quay.io/testcontainers/vnc-recorder:1.1.0
ambassador.container.image = richnorth/ambassador:latest
compose.container.image = docker/compose:1.8.0
kafka.container.image = confluentinc/cp-kafka
localstack.container.image = localstack/localstack
ryuk.container.image = quay.io/testcontainers/ryuk:0.2.3

However, I would like to be able to specify default images using something like this:

tinyimage.container.image.prefix = docker-io.company.com/alpine
vncrecorder.container.image.prefix = quay-io.company.com/testcontainers/vnc-recorder
ambassador.container.image.prefix = docker-io.company.com/richnorth/ambassador
compose.container.image.prefix = docker-io.company.com/docker/compose
kafka.container.image.prefix = docker-io.company.com/confluentinc/cp-kafka
localstack.container.image.prefix = docker-io.company.com/localstack/localstack
ryuk.container.image.prefix = quay-io.company.com/testcontainers/ryuk

Start with docs/features/configuration.md and trace how the documented image properties are resolved for the default images. Add support for image prefixes while preserving the default tags, and verify that the existing full-image overrides continue to work.