$ docker-compose up -d
$ docker exec -it explorer-cli /bin/bash
# pwd
/go
# ls
bin  pkg  src
# echo $GOPATH
/go
# ls /go/pkg/linux_amd64/github.com/
go-sql-driver
# go build -i -o /go/bin/explorer-cli src/main.go
src/main.go:6:2: cannot find package "github.com/go-sql-driver/mysql" in any of:
    /usr/local/go/src/github.com/go-sql-driver/mysql (from $GOROOT)
    /go/src/github.com/go-sql-driver/mysql (from $GOPATH)

(it worked if I run "go get" manually)
# go get github.com/go-sql-driver/mysql
# ls src/
github.com  main.go
# go build -i -o /go/bin/explorer-cli src/main.go

version: '3.4'

services:
  mysql:
    image: mysql:latest
    container_name: database
    volumes:
      - ./docker/:/etc/mysql/conf.d
      - ./docker/:/docker-entrypoint-initdb.d
    environment:
      - MYSQL_RANDOM_ROOT_PASSWORD=true
      - MYSQL_DATABASE=explorer
      - MYSQL_USER=admin
      - MYSQL_PASSWORD=12dlql*41
  app:
    build: .
    tty: true
    image: explorer-cli:latest
    container_name: explorer-cli
    volumes:
      - ./src:/go/src
    external_links:
      - database

FROM golang:latest

RUN apt-get update
RUN apt-get upgrade -y

ENV GOBIN /go/bin

RUN go get github.com/go-sql-driver/mysql

package main

import (
    "database/sql"

    _ "github.com/go-sql-driver/mysql"
)

func main() {
    db, err := sql.Open("mysql", "admin:12dlql*41@(database:3306)/explorer")
    if err != nil {
        panic(err.Error())
    }
    defer db.Close()
}

# ls /go/pkg/linux_amd64/github.com/go-sql-driver/
mysql.a

# ls /go/src/github.com/go-sql-driver/mysql/
AUTHORS         connection_go18_test.go  packets.go
CHANGELOG.md        connection_test.go   packets_test.go
CONTRIBUTING.md     const.go         result.go
LICENSE         driver.go        rows.go
README.md       driver_go18_test.go  statement.go
appengine.go        driver_test.go       statement_test.go
benchmark_go18_test.go  dsn.go           transaction.go
benchmark_test.go   dsn_test.go      utils.go
buffer.go       errors.go        utils_go17.go
collations.go       errors_test.go       utils_go18.go
connection.go       fields.go        utils_go18_test.go
connection_go18.go  infile.go        utils_test.go