Make env in ImageConfig a map

Fields in CUE were renamed to the lowercase version of Dockerfile instructions.

There's now opportunity to make other fields simpler to use (e.g., healthcheck), this commit is focused on env.

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

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

@@ -35,15 +35,28 @@ package engine
 
 // Container image config. See [OCI](https://www.opencontainers.org/).
 // Spec left open on purpose to account for additional fields.
-// [Image Spec](https://github.com/opencontainers/image-spec/blob/main/specs-go/v1/config.go)
-// [Docker Superset](https://github.com/moby/buildkit/blob/master/frontend/dockerfile/dockerfile2llb/image.go)
 #ImageConfig: {
-	Env?: [...string]
-	User?: string
-	Cmd?: [...string]
+	user?: string
+	expose?: [string]: {}
+	env?: [string]: string
+	entrypoint?: [...string]
+	cmd?: [...string]
+	volume?: [string]: {}
+	workdir?: string
+	label?: [string]: string
+	healthcheck?: #HealthCheck
+	shell?: [...string]
 	...
 }
 
+#HealthCheck: {
+	test?: [...string]
+	interval?:    int
+	timeout?:     int
+	startPeriod?: int
+	retries?:     int
+}
+
 // Download a container image from a remote repository
 #Pull: {
 	$dagger: task: _name: "Pull"