File not found when copied in docker container via multistage build

Having a trouble to open a simple file from Golang located in project root when copied in container via Docker multistage build. However I can see the file in container.

Here is my Dockerfile:

# Start from a base Go image
FROM golang:1.19 as builder

# Set the working directory inside the container
WORKDIR /app

# Copy the Go module files
COPY go.mod go.sum ./

# Download the dependencies
RUN go mod download

# Copy the rest of the application code
COPY . .

# Copy the promotions.csv into the container
RUN chmod +r /app/promotions.csv
COPY promotions.csv /app/promotions.csv

# Run with disabled cross-compilation
RUN CGO_ENABLED=0 GOOS=linux go build -o app .

# Final stage
FROM alpine:3.18

RUN apk --no-cache add ca-certificates

WORKDIR /root/

COPY --from=builder --chown=${USERNAME}:${USERNAME} /app/app .

EXPOSE 1321

CMD ["./app"]

promotions.csv file exists though in container file system:

However records, err := readCSVFile("promotions.csv") code from main.go returns an error:

open promotions.csv: no such file or directory

More strangely everything is working fine when I don't use mutistage build:

# Start from a base Go image
FROM golang:1.19 as builder

# Set the working directory inside the container
WORKDIR /app

# Copy the Go module files
COPY go.mod go.sum ./

# Download the dependencies
RUN go mod download

#COPY promotions.csv ./

# Copy the rest of the application code
COPY . .

# Copy the promotions folder into the container

# Run with disabled cross-compilation
RUN go build -o main

EXPOSE 1321

CMD ["./main"]

Here is the docker-compose.yml:

version: '3'

services:
  postgres:
    image: postgres:15.3-alpine
    restart: always
    ports:
      - 5432:5432
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: password
      POSTGRES_DB: vrstore

  app:
    build:
      context: .
      dockerfile: Dockerfile
    restart: always
    ports:
      - 1321:1321
    depends_on:
      - postgres
    volumes:
      - .:/app
    environment:
      DATABASE_HOST: postgres
      DATABASE_PORT: 5432
      DATABASE_NAME: vrstore
      DATABASE_USER: postgres
      DATABASE_PASSWORD: password

What could possibly be the reason of the error in the first case? Maybe I'm missing something obvious? Thanks.

The file does not exist in the image since you don't copy it there.

In the container, you are mounting the files as a volume into /app:

    volumes:
      - .:/app

However you are running the binary from /root:

WORKDIR /root/
COPY --from=builder --chown=${USERNAME}:${USERNAME} /app/app .
CMD ["./app"]

So the binary is looking for /root/promotions.csv and cannot find it. You would need to change directory to /app or open the file with the fully qualified path.