Add pdbmux: merging PuppetDB proxy daemon

Split out from node-lookup PR #17 into its own repo. pdbmux presents a
single merged PuppetDB v4 query surface over the old (Consul) and new
(k8s) PuppetDBs during the VM to k8s migration, and is deployed in-cluster
via argocd-apps as a container image.

Dockerfile

@@ -0,0 +1,24 @@
+# Container image for pdbmux, the merging PuppetDB proxy daemon. pdbmux is a
+# k8s-only service (deployed via argocd-apps), so it ships as a distroless
+# static image rather than an RPM.
+FROM golang:1.25-alpine AS builder
+
+RUN apk add --no-cache git
+
+WORKDIR /build
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+ARG VERSION=dev
+RUN CGO_ENABLED=0 go build -ldflags="-s -w -X main.version=${VERSION}" -o pdbmux .
+
+FROM gcr.io/distroless/static-debian12:nonroot
+
+COPY --from=builder /build/pdbmux /usr/local/bin/pdbmux
+
+EXPOSE 8080
+
+ENTRYPOINT ["pdbmux", "serve"]