{
  "id": "docker-build-image",
  "title": "Write a Real Dockerfile and Build a Real Image",
  "category": "Containers",
  "isolation_type": "shared_namespace",
  "difficulty": "Medium",
  "type": "task",
  "weight": 6,
  "description": "## Write a Real Dockerfile and Build a Real Image\n\nA file `/srv/build-lab/greeting.txt` already exists, containing a message.\n\n**Your task:**\n\n1. Write a `Dockerfile` at `/srv/build-lab/Dockerfile` that:\n   - Starts `FROM alpine:3.19`\n   - Sets the working directory to `/app`\n   - Copies `greeting.txt` into the image\n   - Runs `cat greeting.txt` as its default command\n2. Build it, tagged `rootnode-build-lab:1`\n3. Run a container from YOUR image and confirm it prints the real content of `greeting.txt`",
  "hints": [
    {
      "title": "The Dockerfile",
      "body": "Each instruction builds one layer: FROM sets the base, WORKDIR sets the working directory for everything after it, COPY brings a file from the build context into the image, CMD sets the default command run when a container starts.",
      "command": "cat <<EOF > /srv/build-lab/Dockerfile\nFROM alpine:3.19\nWORKDIR /app\nCOPY greeting.txt .\nCMD cat greeting.txt\nEOF"
    },
    {
      "title": "Building and running",
      "body": "`docker build -t <tag> <context-directory>` builds using the Dockerfile found in that directory. `docker run --rm <tag>` runs it once and cleans up automatically afterward.",
      "command": "cd /srv/build-lab && docker build -t rootnode-build-lab:1 .\ndocker run --rm rootnode-build-lab:1"
    }
  ],
  "setup_commands": [
    { "command": "rm -rf /srv/build-lab && mkdir -p /srv/build-lab" },
    { "command": "echo 'Hello from my custom image' > /srv/build-lab/greeting.txt" },
    { "command": "docker pull alpine:3.19" }
  ],
  "validation": {
    "commands": [
      { "description": "the image was actually built with the expected tag", "command": "docker image inspect rootnode-build-lab:1 --format '{{.RepoTags}}' 2>/dev/null", "expected_output": "rootnode-build-lab:1", "match": "contains" },
      { "description": "a container from the student's own image produces the real file content", "command": "docker run --rm rootnode-build-lab:1 2>/dev/null", "expected_output": "Hello from my custom image", "match": "exact" }
    ]
  },
  "default_namespace": "default",
  "teardown_commands": [
    { "command": "docker rmi rootnode-build-lab:1 2>/dev/null; rm -rf /srv/build-lab" }
  ]
}