I am pretty sure I am making the stupidest mistake ever, but I can’t figure out why I get 502 Bad Gateway error when I run my backend application in Docker when I’m trying to upload a file more than 16Mb in Size.

Dockerfile is pretty straightforward:

FROM golang:1.22.4 AS builder

WORKDIR /build

COPY . .

WORKDIR /build/apps/Myapp/backend

RUN go mod download

ENV CGO_ENABLED=0 GOOS=linux GOARCH=amd64
RUN go build -ldflags='-s -w -extldflags "-static"' -o myapp-api .

FROM alpine as certs
RUN apk --no-cache add ca-certificates

FROM scratch

COPY --from=certs /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
COPY --from=builder ["/build/apps/Myapp/backend/myapp-api", "/build/apps/Myapp/backend/.env.production", "/"]
COPY --from=builder ["/build/apps/Myapp/backend/templates", "/templates"]

EXPOSE 4000

ENTRYPOINT ["/myapp-api"]
CMD ["--mode", "prod"]

My backend is Go + Fiber, and it is configured for big files:

    app := fiber.New(fiber.Config{
        ErrorHandler: utils.ErrorHandler,
        BodyLimit:    520 * 1024 * 1024,
        JSONEncoder:  json.Marshal,
        JSONDecoder:  json.Unmarshal,
    })

And everything works just fine when I run it locally, without Docker. Once it inside Docker, every attempt to upload a file more than 15MB will throw 502 Bad Gateway error. Smaller files can be uploaded just fine even when in Docker.

So it seems like some kind of a limitation on Docker side.