two versions, one based on README and another based on removing eslint warnings

gatsby-node-assign.js

export function modifyWebpackConfig (config) {
  // Object.assign is not picked up as mutation by immutable linter ?!
  config.loader('jpg', (cfg) => Object.assign(
    cfg,
    {
      test: /\.jpe?g$|\.gif$|\.png$/i,
      loader: 'file-loader?name=images/[name].[ext]',
    }
  ))
  return config
}

gatsby-node-spread.js

export function modifyWebpackConfig (config) {
  // arrow-body-style rule doesn't recognize that the spread operator needs
  // to be in an object literal
  // eslint-disable-next-line arrow-body-style
  config.loader('jpg', (cfg) => {
    return {
      ...cfg,
      test: /\.jpe?g$|\.gif$|\.png$/i,
      loader: 'file-loader?name=images/[name].[ext]',
    }
  })
  return config
}

gatsby-node.js

// eslint-disable-next-line immutable/no-mutation,no-unused-vars
exports.modifyWebpackConfig = function (config, env) {
  config.loader('jpg', (cfg) => {
    // eslint-disable-next-line immutable/no-mutation,no-param-reassign
    cfg.test = /\.jpe?g$|\.gif$|\.png$/i
    // eslint-disable-next-line immutable/no-mutation,no-param-reassign
    cfg.loader = 'file-loader?name=images/[name].[ext]'
    return cfg
  })
  return config
}