bake: deduplicate context transfer use case

Signed-off-by: CrazyMax <[email protected]>

content/build/bake/contexts.md

@@ -88,3 +88,49 @@ target "app" {
 In most cases you should just use a single multi-stage Dockerfile with multiple
 targets for similar behavior. This case is only recommended when you have
 multiple Dockerfiles that can't be easily merged into one.
+
+## Deduplicate context transfer
+
+When building multiple targets using the same context, transferring the context
+for each target can be inefficient. To avoid transferring the same context
+multiple times, use a named context and reference it for each target.
+
+```dockerfile
+FROM alpine AS base
+WORKDIR /work
+
+FROM scratch AS ctx
+COPY --link . .
+
+FROM base AS target1
+RUN apk add git
+RUN --mount=from=ctx ls -l
+
+FROM base AS target2
+RUN apk add curl
+RUN --mount=from=ctx ls -l
+```
+
+```hcl
+group "default" {
+  targets = ["target1", "target2"]
+}
+
+target "ctx" {
+  target = "ctx"
+}
+
+target "target1" {
+  target = "target1"
+  contexts = {
+    ctx = "target:ctx"
+  }
+}
+
+target "target2" {
+  target = "target2"
+  contexts = {
+    ctx = "target:ctx"
+  }
+}
+```