Add docker.#Set for updating image metadata

Signed-off-by: Helder Correia <174525+helderco@users.noreply.github.com>

pkg/dagger.io/dagger/engine/image.cue

@@ -1,5 +1,9 @@
 package engine
 
+import (
+	"list"
+)
+
 // Upload a container image to a remote repository
 #Push: {
 	$dagger: task: _name: "Push"
@@ -34,7 +38,6 @@ package engine
 #Ref: string
 
 // Container image config. See [OCI](https://www.opencontainers.org/).
-// Spec left open on purpose to account for additional fields.
 #ImageConfig: {
 	user?: string
 	expose?: [string]: {}
@@ -116,3 +119,75 @@ package engine
 	// Container image config produced
 	config: #ImageConfig
 }
+
+// Change image config
+#Set: {
+	// The source image config
+	input: #ImageConfig
+
+	// The config to merge
+	config: #ImageConfig
+
+	// Resulting config
+	output: #ImageConfig & {
+		let structs = ["env", "label", "volume", "expose"]
+		let lists = ["onbuild"]
+
+		// doesn't exist in config, copy away
+		for field, value in input if config[field] == _|_ {
+			"\(field)": value
+		}
+
+		// only exists in config, just copy as is
+		for field, value in config if input[field] == _|_ {
+			"\(field)": value
+		}
+
+		// these should exist in both places
+		for field, value in config if input[field] != _|_ {
+			"\(field)": {
+				// handle structs that need merging
+				if list.Contains(structs, field) {
+					_#mergeStructs & {
+						#a: input[field]
+						#b: config[field]
+					}
+				}
+
+				// handle lists that need concatenation
+				if list.Contains(lists, field) {
+					list.Concat([
+						input[field],
+						config[field],
+					])
+				}
+
+				// replace anything else
+				if !list.Contains(structs+lists, field) {
+					value
+				}
+			}
+		}
+	}
+}
+
+// Merge two structs by overwriting or adding values
+_#mergeStructs: {
+	// Struct with defaults
+	#a: [string]: _
+
+	// Struct with overrides
+	#b: [string]: _
+	{
+		// FIXME: we need exists() in if because this matches any kind of error (cue-lang/cue#943)
+		// add anything not in b
+		for field, value in #a if #b[field] == _|_ {
+			"\(field)": value
+		}
+
+		// safely add all of b
+		for field, value in #b {
+			"\(field)": value
+		}
+	}
+}

pkg/universe.dagger.io/docker/set.cue

@@ -0,0 +1,25 @@
+package docker
+
+import (
+	"dagger.io/dagger/engine"
+)
+
+// Change image config
+#Set: {
+	// The source image
+	input: #Image
+
+	// The image config to change
+	config: engine.#ImageConfig
+
+	_set: engine.#Set & {
+		"input":  input.config
+		"config": config
+	}
+
+	// Resulting image with the config changes
+	output: #Image & {
+		rootfs: input.rootfs
+		config: _set.output
+	}
+}

pkg/universe.dagger.io/docker/test/docker.bats

@@ -22,3 +22,7 @@ setup() {
     dagger up ./run-export-directory-test.cue
     dagger up ./image-config-test.cue
 }
+
+@test "docker.#Set" {
+    dagger up ./set.cue
+}

pkg/universe.dagger.io/docker/test/set.cue

@@ -0,0 +1,40 @@
+package docker
+
+import (
+	"dagger.io/dagger"
+	"dagger.io/dagger/engine"
+	"universe.dagger.io/docker"
+)
+
+dagger.#Plan & {
+	actions: {
+		image: output: docker.#Image & {
+			rootfs: engine.#Scratch
+			config: {
+				cmd: ["/bin/sh"]
+				env: PATH: "/sbin:/bin"
+				onbuild: ["COPY . /app"]
+			}
+		}
+		set: docker.#Set & {
+			input: image.output
+			config: {
+				env: FOO: "bar"
+				workdir: "/root"
+				onbuild: ["RUN /app/build.sh"]
+			}
+		}
+		verify: set.output.config & {
+			env: {
+				PATH: "/sbin:/bin"
+				FOO:  "bar"
+			}
+			cmd: ["/bin/sh"]
+			workdir: "/root"
+			onbuild: [
+				"COPY . /app",
+				"RUN /app/build.sh",
+			]
+		}
+	}
+}