create backfill for transcoding media particles

Closes #192

go/Dockerfile.transcodebackfill

@@ -0,0 +1,22 @@
+# golang two stage build
+FROM golang:1.25 AS first-stage
+
+WORKDIR /app
+
+COPY go.mod go.sum ./
+RUN go mod download && go mod verify
+
+COPY . .
+
+WORKDIR /app/cmd/transcodebackfill
+RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -o main
+RUN ls
+
+FROM alpine:latest AS second-stage
+
+RUN apk add --no-cache ffmpeg ca-certificates
+
+WORKDIR /app
+COPY --from=first-stage /app/cmd/transcodebackfill .
+RUN echo "copied over binary to production stage"
+CMD ["./main"]