docs: bake guides and refactor reference

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/guides/bake/build-contexts.md

@@ -0,0 +1,77 @@
+---
+title: "Defining additional build contexts and linking targets"
+keywords: build, buildx, bake, buildkit, hcl
+---
+
+In addition to the main `context` key that defines the build context each target
+can also define additional named contexts with a map defined with key `contexts`.
+These values map to the `--build-context` flag in the [build command](https://docs.docker.com/engine/reference/commandline/buildx_build/#build-context).
+
+Inside the Dockerfile these contexts can be used with the `FROM` instruction or `--from` flag.
+
+The value can be a local source directory, container image (with `docker-image://` prefix),
+Git URL, HTTP URL or a name of another target in the Bake file (with `target:` prefix).
+
+## Pinning alpine image
+
+```dockerfile
+# syntax=docker/dockerfile:1
+FROM alpine
+RUN echo "Hello world"
+```
+
+```hcl
+# docker-bake.hcl
+target "app" {
+  contexts = {
+    alpine = "docker-image://alpine:3.13"
+  }
+}
+```
+
+## Using a secondary source directory
+
+```dockerfile
+# syntax=docker/dockerfile:1
+FROM scratch AS src
+
+FROM golang
+COPY --from=src . .
+```
+
+```hcl
+# docker-bake.hcl
+target "app" {
+  contexts = {
+    src = "../path/to/source"
+  }
+}
+```
+
+## Using a result of one target as a base image in another target
+
+To use a result of one target as a build context of another, specity the target
+name with `target:` prefix.
+
+```dockerfile
+# syntax=docker/dockerfile:1
+FROM baseapp
+RUN echo "Hello world"
+```
+
+```hcl
+# docker-bake.hcl
+target "base" {
+  dockerfile = "baseapp.Dockerfile"
+}
+
+target "app" {
+  contexts = {
+    baseapp = "target:base"
+  }
+}
+```
+
+Please note that in most cases you should just use a single multi-stage
+Dockerfile with multiple targets for similar behavior. This case is recommended
+when you have multiple Dockerfiles that can't be easily merged into one.