Being new to docker files. I am trying to understand how docker build is working here. I currently have a folder that has two files in it. The folder is called foo The structure of the folder is as follows. It has two files in it the first file is main.go and the other file next to it is Dockerfile as shown in the diagram below

Foo
|_main.go
|_go.mod
|_go.sum
|_Dockerfile

My Dockerfile currently looks like this and the command docker build -t .. is called from inside the folder Foo.

FROM golang:1.18-alpine AS builder

# Create and change to the app directory.
WORKDIR /app

COPY go.* ./
RUN go mod download

# Copy local code to the container image.
COPY . ./

RUN go build -v -o my app
....

Now this is a fairly simple Dockerfile. Here is what I understand.

1- golang:1.18-alpine is the base image

2- In the container a folder called app will be created

3- go.mod and go.sum will be copied to ./ path of the container (probably home) of the container.

4- Run go mod download will be called from inside the /app folder correct ?

My question is basically for no. 4. go mod download called from inside /app folder ? If so how does that work because from my understanding is that the /app folder is so far empty ? When did the app folder get populated ? In the statement COPY go.* ./ what path is ./ is that the home ?

CodePudding user response：

In line WORKDIR /app, your current path will be set to /app. If the directory don't exist then it will be created beforehand.

Next COPY go.* ./, this matches all of files start with go. will be copied to /app directory in the docker container. So your docker /app should look like this :

/app
| go.mod
| go.sum

Again with COPY . ./, you are copying all files from current directory to /app directory of your docker container. It will replace already existing files in the contrainer. /app will look like this:

/app
| main.go
| go.mod
| go.sum
| Dockerfile

Last with RUN go build -v -o myapp, you are building the app using go and saving binary file myapp.