/**
 * Runs npm install inside current container.
 * Must be called inside container('node') block.
 *
 * config keys (all optional):
 *   workDir - directory to run in, default: '.'
 */
def call(Map config = [:]) {
    def workDir = config.workDir ?: '.'
    dir(workDir) {
        sh 'npm install'
    }
}