feat: add custom container registry substitutor

[stillya]

Custom Registry Substitutor

What does this PR do?

The new substitutor can be used by modules whose images are not placed on DockerHub by default. This way, you don't need to manually prefix your image with a custom container registry. Instead, you can configure your registry with a prefix at the configuration level using hub.image.name.prefix.

In a development environment, hub.image.name.prefix is commonly empty, so CustomHubSubstitutor will add the desired default prefix. However, in a CI environment, the default prefix will not be added, and the config prefix will be used (handled by prependHubRegistry).

Usage

func Run(ctx context.Context, img string, opts ...testcontainers.ContainerCustomizer) (*KeycloakContainer, error) {
	req := testcontainers.ContainerRequest{
		Image: img,
		Env: map[string]string{
			keycloakAdminUsernameEnv: defaultKeycloakAdminUsername,
			keycloakAdminPasswordEnv: defaultKeycloakAdminPassword,
		},
		ExposedPorts: []string{keycloakPort},
		Cmd:          []string{keycloakStartupCommand},
		ImageSubstitutors: []testcontainers.ImageSubstitutor{
			testcontainers.NewCustomHubSubstitutor("quay.io"),
		},
	}
}

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[mdelapenya]

@stillya thanks for submitting the PR. After discussing this on Slack, and seeing the code, I think the substitutor could make sense. I have one question regarding

In a development environment, hub.image.name.prefix is commonly empty, so CustomHubSubstitutor will add the desired default prefix. However, in a CI environment, the default prefix will not be added, and the config prefix will be used (handled by prependHubRegistry).

Do you have issues updating the properties file on the CI, configuring the well-known environment in which the project must work? I think it would be possible to do:

echo "hub.image.name.prefix=quay.io" >> ~/.testcontainers.properties 

as part of your Github action job, jenkins job, etc.

I'd like to understand the reasons why it's not possible to do that. Of course, if it's in code, the settings are applied to everybody, that's why I consider this changeset correct. Just want verify you have a workaround in the meantime.

[mdelapenya]

Responding to myself (thanks @kiview): setting the properties will apply to all containers, but the substitutor will apply to the ones of interest.

[stillya]

Yep, that's the reason.

[mdelapenya]

LGTM, thanks!