linuxkit pkg: Add config field to build.yml

This is a `moby.ImageConfig` struct which is marshalled into JSON and added as
the `org.mobyproject.config` label on the built image.

Convert `pkg/sysctl` as PoC.

Signed-off-by: Ian Campbell <ijc@docker.com>

pkg/sysctl/Dockerfile

@@ -13,4 +13,3 @@ WORKDIR /
 COPY --from=mirror /go/bin/sysctl /usr/bin/sysctl
 COPY etc/ /etc/
 CMD ["/usr/bin/sysctl"]
-LABEL org.mobyproject.config='{"pid": "host", "readonly": true, "capabilities": ["CAP_SYS_ADMIN"]}'

pkg/sysctl/build.yml

@@ -1 +1,6 @@
 image: sysctl
+config:
+  pid: "host"
+  readonly: true
+  capabilities:
+    - CAP_SYS_ADMIN