If you use

docker.buildAndPushImage()

to biuld the image before pushing to the registry, the first parameter is your image name

This is a function I wrote in TS

private computeImageFromAsset(appPath: string, repositoryUrl: string, registryId: string, parent: pulumi.Resource): pulumi.Output<string> {

        let imageName = this.args.imageName;

        let dockerBuildArgs: any = {};

        let _dockerBuild: docker.DockerBuild = {
            args: dockerBuildArgs,
            context: appPath,
            dockerfile: `${appPath}/Dockerfile`,
            cacheFrom: {},
        };

        const uniqueImageName = docker.buildAndPushImage(imageName, _dockerBuild, repositoryUrl, parent, async () => {

            let credentials = await aws.ecr.getCredentials({
                registryId: registryId
            }, {
                parent, async: true
            });

            let decodedCredentials = Buffer.from(credentials.authorizationToken, "base64").toString();
            let [username, password] = decodedCredentials.split(":");
            if (!password || !username) {
                throw new Error("Invalid credentials");
            }

            return {
                registry: credentials.proxyEndpoint,
                username: username,
                password: password,
            };
        });

        uniqueImageName.apply(d => pulumi.log.debug(`    build complete: ${imageName} (${d})`, parent));
        return uniqueImageName;
    }