Add Dockerfiles and CI pipeline for Docker build/push

- Dockerfile.cli: multi-stage build for exploredns CLI tool
- Dockerfile.web: multi-stage build for the HTTP API/web server
- Both use golang:1.24-alpine builder + alpine:3.21 final image
- CI pipeline docker job builds and pushes to gitea.hansenits.com.au registry on push to main/master

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

Dockerfile.cli

@@ -0,0 +1,20 @@
+# Build stage
+FROM golang:1.24-alpine AS builder
+
+WORKDIR /src
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+RUN CGO_ENABLED=0 GOOS=linux go build -trimpath -ldflags="-s -w" -o /out/exploredns ./cmd/exploredns
+
+# Final stage
+FROM alpine:3.21
+
+RUN apk --no-cache add ca-certificates
+
+COPY --from=builder /out/exploredns /usr/local/bin/exploredns
+
+ENTRYPOINT ["exploredns"]