docs: add the CI use case

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

docs/use-cases/1012-ci.md

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+---
+slug: /1012/ci
+---
+
+# Continuous Integration
+
+Dagger is the perfect tool for CI workflows.
+
+## Benefits
+
+- **Develop and run your CI pipeline locally.** No need to create a Pull Request
+  to trigger CI, you can run the pipeline locally. Since dagger workflows
+  are containerized, you can expect the same results no matter where the pipeline
+  is executed.
+- **Write once, Run anywhere.** The same pipeline can run in any CI, goodbye
+  vendor lock in.
+- **Blazing Fast**: Dagger will automatically build an optimized execution
+  graph, completing the job as fast as possible. Spend less time staring at CI to
+  complete and reduce costs.
+- **Effortless Cache Optimizations**. Dagger will automatically
+  re-use cached execution results if no changes are detected. Made a change to the
+  frontend? The backend won't be built again.
+- **Composable & Reusable**. Define re-usable steps and [share](../learn/1010-dev-cue-package.md) them across all
+  your projects. Or with the world. Dagger
+  ships with [dozens of re-usable components](../reference/universe/README.md)
+
+## Example
+
+This example illustrates how to use Dagger to test and lint a Go project. The
+use of Go is irrelevant and the example can be easily adapted to other languages.
+
+From the project repository, create a file named `ci/main.cue` and add the
+following configuration to it.
+
+```cue title="ci/main.cue"
+package main
+
+import (
+    "alpha.dagger.io/dagger"
+    "alpha.dagger.io/os"
+    "alpha.dagger.io/docker"
+)
+
+// Source directory of the repository. We'll connect this from the CLI using `dagger input`
+source: dagger.#Artifact
+
+// Here we define a test phase.
+// We're using `os.#Container`, a built-in package that will spawn a container
+// We're also using `docker.#Pull` to execute the container from a Docker image
+// comifrom a registry.
+test: os.#Container & {
+    image: docker.#Pull & {
+        from: "golang:1.16-alpine"
+    }
+    mount: "/app": from: source
+    command: "go test -v ./..."
+    dir:     "/app"
+}
+
+// Likewise, here we define a lint phase.
+lint: os.#Container & {
+    image: docker.#Pull & {
+        from: "golangci/golangci-lint:v1.39.0"
+    }
+    mount: "/app": from: source
+    command: "golangci-lint run -v"
+    dir:     "/app"
+}
+```
+
+The configuration above defines:
+
+- **source** code of the project. More on this later.
+- **test** *task* which executes `go test` inside the source artifact
+  using the `golang` Docker image
+- **lint** *task* which executes `golangci-lint` inside the source artifact
+  using the `golangci-lint` Docker image.
+
+Before we can execute the configuration, we need to set up the Dagger workspace and environment.
+
+```shell
+# Initialize a dagger workspace at the root of your project
+dagger init
+
+# Create a CI environment using the CUE files in the `./ci` directory
+dagger new ci -p ./ci
+
+# Link the `source` artifact defined in the configuration with the project
+# source code.
+dagger input dir source .
+```
+
+Next, bring up the CI environment (e.g. execute the CI configuration):
+
+```shell
+$ dagger up
+# ...
+7:15PM INF test | computing    environment=ci
+7:15PM INF lint | computing    environment=ci
+# ...
+```
+
+Since `test` and `lint` do not depend on each other, they are executed in
+parallel.
+
+Running `dagger up` a second time will return almost immediately: since no
+changes were made to `source`, Dagger will re-use the cached results for both `test` nor `lint`.
+
+## Integrating with CI
+
+All it takes to execute a Dagger workflow in CI is to run `dagger up`.
+
+We provide a [GitHub Actions](../learn/1009-github-actions.md) to make
+integration with GitHub easier.
+
+## Monorepos
+
+Dagger workflows scale really well with CI complexity.
+
+The following example illustrates how to test two different projects in the same
+repository: a *Node* frontend (located in *./frontend*) along with a *Go* backend (located in *./backend*).
+
+From the project repository, update the file named `ci/main.cue` with the
+following configuration.
+
+```cue title="ci/main.cue"
+package main
+
+import (
+    "alpha.dagger.io/dagger"
+    "alpha.dagger.io/os"
+    "alpha.dagger.io/docker"
+)
+
+// Source directory of the repository. We'll connect this from the CLI using `dagger input`
+source: dagger.#Artifact
+
+backend: {
+    // We use `os.#Dir` to grab a sub-directory of `source`
+    code: os.#Dir & {
+        from: source
+        path: "."
+    }
+
+    test: os.#Container & {
+        image: docker.#Pull & {
+            from: "golang:1.16-alpine"
+        }
+        mount: "/app": from: code
+        command: "go test -v ./..."
+        dir:     "/app"
+    }
+}
+
+frontend: {
+    // We use `os.#Dir` to grab a sub-directory of `source`
+    code: os.#Dir & {
+        from: source
+        path: "./frontend"
+    }
+
+    test: os.#Container & {
+        image: docker.#Pull & {
+            from: "node:16-alpine"
+        }
+        mount: "/app": from: code
+        command: """
+            # Install Dependencies
+            yarn
+
+            # Run the test script
+            yarn test
+            """
+        dir: "/app"
+    }
+}
+```
+
+:::tip
+Larger configurations can be split into multiple files, for example `backend.cue` and `frontend.cue`
+:::
+
+The configuration above defines a *frontend* and *backend* structure, each
+containing:
+
+- A **code** directory, defined as a subdirectory of **source**
+- A language specific **test** task using either `yarn` or `go`
+
+```shell
+$ dagger up
+7:15PM INF frontend.test | computing    environment=ci
+7:15PM INF backend.test | computing    environment=ci
+7:15PM INF frontend.test | #8 0.370 yarn install v1.22.5    environment=ci
+7:15PM INF frontend.test | #8 0.689 [1/4] Resolving packages...    environment=ci
+7:15PM INF frontend.test | #8 1.626 [2/4] Fetching packages...    environment=ci
+...
+```
+
+`frontend.test` and `backend.test` are running in parallel since there are no
+dependencies between each other.
+
+If you were to make changes to the *./frontend* directory, only
+`frontend.test` will be executed.

docs/use-cases/_category_.json

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+{
+  "label": "Use Cases",
+  "position": 2,
+  "collapsed": false
+}