build: Bump gpg to alpine's edge

LibKSBA is a dependency to GnuPG, which has a CVE that
is yet to be patched on Alpine's stable channel. This
PR installs GnuPG from the edge channel, and should be
reverted once libksba's version 1.6.2 is in main.

https://pkgs.alpinelinux.org/packages?name=libksba&branch=edge
https://gnupg.org/blog/20221017-pepe-left-the-ksba.html

Signed-off-by: Paulo Gomes <paulo.gomes@weave.works>

Dockerfile

@@ -33,7 +33,9 @@ RUN xx-go build -trimpath -a -o kustomize-controller main.go
 
 FROM alpine:3.16
 
-RUN apk add --no-cache ca-certificates tini git openssh-client gnupg
+# Uses GnuPG from edge to patch CVE-2022-3515.
+RUN apk add --no-cache ca-certificates tini git openssh-client && \
+	apk add --no-cache gnupg --repository=https://dl-cdn.alpinelinux.org/alpine/edge/main
 
 COPY --from=builder /workspace/kustomize-controller /usr/local/bin/