gatsby-plugin-sitemap configuration for markdown pages with lastmod

gatsby-config.js

 {
    resolve: `gatsby-plugin-sitemap`,
    options: {
      query: `{
        site {
          siteMetadata {
            siteUrl
          }
        }
        allSitePage {
          nodes {
            path
          }
        }
        allMarkdownRemark {
          edges {
            node {
              fields {
                slug
              }
              frontmatter {
                date(formatString: "YYYY-MM-DD")
              }
            }
          }
        }        
      }`,
      resolvePages: ({
        allSitePage: { nodes: allPages },
        allMarkdownRemark: { edges: allMarkdownEdges },
      }) => {
        const markdownNodeMap = allMarkdownEdges.reduce((acc, edge) => {
          acc[edge.node.fields.slug] = edge.node.frontmatter
          return acc
        }, {})
        return allPages.map(page => {
          return { ...page, ...markdownNodeMap[page.path] }
        })
      },
      serialize: ({ path, date }) => {
        return {
          url: path,
          changefreq: `daily`,
          priority: 0.7,
          lastmod: date,
        }
      },
    },
  },