drop capabilities, enable seccomp and enforce runAsNonRoot

Further restricts the SecurityContext that the controller runs under, by enabling the default seccomp profile and dropping all linux capabilities.
This was set at container-level to ensure backwards compatibility with use cases in which sidecars are injected into the source-controller pod
without setting less restrictive settings.
Add a uid and gid for the container to enforce runAsNonRoot and ensure
the use of non root users.

BREAKING CHANGES:
1) The use of new seccomp API requires Kubernetes 1.19.
2) the controller container is now executed under 65534:65534 (userid:groupid).
   This change may break deployments that hard-coded the user name 'controller' in their PodSecurityPolicy.

Signed-off-by: Sanskar Jaiswal<sanskar.jaiswal@weave.works>
Co-authored-by: Paulo Gomes <paulo.gomes@weave.works>

Dockerfile

@@ -43,9 +43,7 @@ COPY --from=builder /workspace/kustomize-controller /usr/local/bin/
 # https://github.com/gliderlabs/docker-alpine/issues/367#issuecomment-354316460
 RUN [ ! -e /etc/nsswitch.conf ] && echo 'hosts: files dns' > /etc/nsswitch.conf
 
-RUN addgroup -S controller && adduser -S controller -G controller
-
-USER controller
+USER 65534:65534
 
 ENV GNUPGHOME=/tmp
 

config/manager/deployment.yaml

@@ -29,6 +29,12 @@ spec:
         securityContext:
           allowPrivilegeEscalation: false
           readOnlyRootFilesystem: true
+          runAsNonRoot: true
+          capabilities:
+            drop: [ "ALL" ]
+          seccompProfile:
+            type: RuntimeDefault
+
         ports:
           - containerPort: 8080
             name: http-prom