Proper support for Docker Image metadata

- Both FetchContainer and DockerBuild read the image metadata and
  convert to LLB (e.g. `ENV foo bar` in Dockerfile shows up in
  `op.#Exec`)
- Image metadata is "sticky" between Pipelines (e.g. `op.#Load` will
  re-use the same metadata)
- Image metadata is injected back to #PushContainer, so that
  DockerBuild+PushContainer and FetchContainer+PushContainer do not lose
  any metadata.
- Tests for all the above

Fixes #142

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

tests/llb/dockerbuild/main.cue

@@ -91,3 +91,32 @@ TestBuildPlatform: #up: [
 		platforms: ["linux/amd64"]
 	},
 ]
+
+TestImageMetadata: #up: [
+	op.#DockerBuild & {
+		dockerfile: """
+			FROM alpine:latest@sha256:ab00606a42621fb68f2ed6ad3c88be54397f981a7b70a79db3d1172b11c4367d
+			ENV CHECK foobar
+			ENV DOUBLECHECK test
+			"""
+	},
+	op.#Exec & {
+		args: ["sh", "-c", #"""
+			env
+			test "$CHECK" = "foobar"
+			"""#]
+	},
+]
+
+// Make sure the metadata is carried over with a `Load`
+TestImageMetadataIndirect: #up: [
+	op.#Load & {
+		from: TestImageMetadata
+	},
+	op.#Exec & {
+		args: ["sh", "-c", #"""
+			env
+			test "$DOUBLECHECK" = "test"
+			"""#]
+	},
+]