Add docs

Author: ofhouse

packages/runtime/docs/runtime.md

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+# Runtime development sheet
+
+From: 20. November 2021
+
+This is a description how the runtime internally works.
+
+## Next.js milestones
+
+### [10.0.9-canary.4](https://github.com/vercel/next.js/releases/tag/v10.0.9-canary.4) - 09 Mar 2021
+
+Beginning with this version the builder no longer sets the environment variable `NEXT_PRIVATE_TARGET=experimental-serverless-trace` and uses the default `target: server`.
+
+### [10.0.8-canary.15](https://github.com/vercel/next.js/releases/tag/v10.0.8-canary.15) - 03 Mar 2021
+
+Beginning with this version Next.js uses the `target: server` by default instead of `target: experimental-serverless-trace`.  
+This can be overridden by setting the environment variable `NEXT_PRIVATE_TARGET=experimental-serverless-trace` in newer versions of Next.js.  
+The path of the server output folder also changed from `.serverless` to `.server` in the build output.
+
+### [9.0.4-canary.1](https://github.com/vercel/next.js/releases/tag/v9.0.4-canary.1) - 06 Aug 2019
+
+Beginning with with this version, the builder uses `target: experimental-serverless-trace` instead of `target: serverless` when building Next.js.
+
+## Procedure
+
+### 1. Download of files
+
+All source files that are required for the build, are downloaded to a temporary folder before the actual build happens.
+Some third-party packages try to detect a Vercel-like build environment like this, so we need to make sure that the following environment variables are set before staring the build:
+
+```
+NOW_BUILDER=1
+VERCEL=1
+```
+
+Since the build script is commonly started from outside of the temporary download folder, we have to manually set the `INIT_CWD` environment variable to the temporary download folder.
+
+### 2. Pre-Build
+
+Before running the build, the `package.json` gets customized to ensure that the `next build` command is executed by the builder.
+If the `build` script is not set, it gets overridden with `next build`.
+When the `build` script is already set, then the `build` script is renamed and runs before the actual Next.js build.
+
+It is then detecting if npm or yarn is used and runs the install command of the package manager that is used.
+
+#### Create serverless `next.config.js`
+
+> This step is only executed on Next.js versions `< 10.0.9-canary.4`.
+
+To set the target option in the `next.config.js`, the original file (if exists) is renamed to `next.config.__vercel_builder_backup__.js`.
+Then a new `next.config.js` is created with the following content:
+
+```js
+module.exports = function (...args) {
+  let original = require('./next.config.__vercel_builder_backup__');
+
+  const finalConfig = {};
+  const target = { target: 'experimental-serverless-trace' };
+
+  if (
+    typeof original === 'function' &&
+    original.constructor.name === 'AsyncFunction'
+  ) {
+    // AsyncFunctions will become promises
+    original = original(...args);
+  }
+
+  if (original instanceof Promise) {
+    // Special case for promises, as it's currently not supported
+    // and will just error later on
+    return original
+      .then((orignalConfig) => Object.assign(finalConfig, orignalConfig))
+      .then((config) => Object.assign(config, target));
+  } else if (typeof original === 'function') {
+    Object.assign(finalConfig, original(...args));
+  } else if (typeof original === 'object') {
+    Object.assign(finalConfig, original);
+  }
+
+  Object.assign(finalConfig, target);
+
+  return finalConfig;
+};
+```