Docker: "go.mod file not found" when it obviously exists

Docker is a popular containerization technology, but sometimes you encounter strange problems when building images. One of the common problems is a "go.mod file not found" error message during the build process, even though the file obviously exists. This issue can be confusing, but there are actually several possible causes and solutions. In this article, we will explore this problem and provide you with some solutions. PHP editor Baicao will help you understand and solve this problem so that your Docker build goes smoothly.

Question content

I'm trying to make a full stack application using go backend and nextjs frontend (which I haven't created yet). I'm following the tutorial for making the backend and so far everything is working fine. However, I want to move the backend stuff into its own folder so I can keep it separate from the frontend. Now whenever I run docker compose build it completes without a hitch, but when I run docker compose up this is what I get (I am using air for thermal Overload function):

goapp-1  | watching .
goapp-1  | watching backend
goapp-1  | watching backend/cmd
goapp-1  | watching backend/cmd/database
goapp-1  | watching backend/cmd/handlers
goapp-1  | watching backend/cmd/models
goapp-1  | watching backend/tmp
goapp-1  | !exclude tmp
goapp-1  | building...
goapp-1  | go: go.mod file not found in current directory or any parent directory; see 'go help modules'
goapp-1  | failed to build, error: exit status 1

But when I go into Docker Desktop and run ls in that image, it says go.mod is there!

So where is the problem?

I tried changing the target directory and context in both compose.yaml and my dockerfile, without success. Running go build instead of air also gives me the same error.

This is my file tree:

databeis2
 ┣ .vscode
 ┃ ┗ settings.json
 ┣ backend
 ┃ ┣ cmd
 ┃ ┃ ┣ ...
 ┃ ┃ ┣ main.go
 ┃ ┃ ┗ routes.go
 ┃ ┣ tmp
 ┃ ┃ ┣ build-errors.log
 ┃ ┃ ┗ main
 ┃ ┣ .air.toml
 ┃ ┣ Dockerfile
 ┃ ┣ go.mod
 ┃ ┣ go.sum
 ┃ ┗ pre_cmd.txt
 ┣ tmp
 ┃ ┗ build-errors.log
 ┣ .env
 ┗ compose.yaml

My docker file:

FROM golang:1.21

WORKDIR /app

RUN go install github.com/cosmtrek/air@latest

COPY . .
RUN go mod tidy

and compose.yaml:

<code>services:
  goapp:
    build:
      context: ./backend
    env_file:
      - .env
    ports:
      - 8000:8000
    volumes:
      - .:/app
    command: air ./cmd/main.go -b 0.0.0.0
    depends_on:
      - db

  db:
    image: postgres:alpine
    environment:
      - POSTGRES_USER=${DB_USER}
      - POSTGRES_PASSWORD=${DB_PASSWORD}
      - POSTGRES_DB=${DB_NAME}
    ports:
      - 5432:5432
    volumes:
      - postgres-db:/var/lib/postgresql/data

volumes:
  postgres-db:
</code>

Any help would be greatly appreciated! !

Solution

I solved it! The problem is that after copying the files in the dockerfile, I need to change WORKDIR to backend .

FROM golang:1.21

WORKDIR /app

RUN go install github.com/cosmtrek/air@latest

COPY . .
# Add the below line 
WORKDIR /app/backend

RUN go mod tidy

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