[FEATURE REQ] Support docker image setting for function app slot

[weidongxu-microsoft]

Request from Hanxiao.

Support docker image setting for function app slot.

        /**
         * A function app definition allowing docker image source to be specified.
         */
        interface WithDockerContainerImage {
            /**
             * Specifies the docker container image to be a built in one.
             * @param runtimeStack the runtime stack installed on the image
             * @return the next stage of the definition
             */
            WithCreate withBuiltInImage(FunctionRuntimeStack runtimeStack);

            /**
             * Specifies the docker container image to be one from Docker Hub.
             * @param imageAndTag image and optional tag (eg 'image:tag')
             * @return the next stage of the definition
             */
            WithCreate withPublicDockerHubImage(String imageAndTag);

            /**
             * Specifies the docker container image to be one from Docker Hub.
             * @param imageAndTag image and optional tag (eg 'image:tag')
             * @return the next stage of the definition
             */
            WithCredentials withPrivateDockerHubImage(String imageAndTag);

            /**
             * Specifies the docker container image to be one from a private registry.
             * @param imageAndTag image and optional tag (eg 'image:tag')
             * @param serverUrl the URL to the private registry server
             * @return the next stage of the definition
             */
            WithCredentials withPrivateRegistryImage(String imageAndTag, String serverUrl);
        }

[weidongxu-microsoft]

@Flanker32

Task to support docker for function app slot.

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