World D2: Recipes

WORKDIR, RUN, ENV and EXPOSE

You will learn: four more recipe lines, and why EXPOSE does not open the hatch.

Checked with Docker Engine 29 on 31 Oct 2026

The problem

A recipe says COPY server.js . and nothing else. The node base sets no work folder. Where does server.js land?

Watch

~/api $ cat DockerfileFROM node:24-alpineWORKDIR /appCOPY package.json package-lock.json ./RUN npm ci --omit=devCOPY server.js .ENV PORT=3000EXPOSE 3000CMD ["node", "server.js"]~/api $ docker build -t api:1.0 .[+] Building 1.3s (10/10) FINISHED                                docker:default => [internal] load build definition from Dockerfile                        0.0s => => transferring dockerfile: 200B                                        0.0s => [internal] load metadata for docker.io/library/node:24-alpine           0.0s => [internal] load .dockerignore                                           0.0s => => transferring context: 2B                                             0.0s => [1/5] FROM docker.io/library/node:24-alpine@sha256:8a32c8cf649a49915a6  0.0s => => resolve docker.io/library/node:24-alpine@sha256:8a32c8cf649a49915a6  0.0s => [2/5] WORKDIR /app                                                      0.0s => [internal] load build context                                           0.0s => => transferring context: 675B                                           0.0s => [3/5] COPY package.json package-lock.json ./                            0.0s => [4/5] RUN npm ci --omit=dev                                             1.2s => [5/5] COPY server.js .                                                  0.0s => exporting to image                                                      0.1s => => exporting layers                                                     0.0s => => exporting manifest sha256:585b1f71ac0676785e33f0a3e3cc27827b1895c98  0.0s => => exporting config sha256:750e26492fb9c3a3ec58b7ad77189f271fe577845a2  0.0s => => exporting attestation manifest sha256:27da90d7c6fda90a44952daaae018  0.0s => => exporting manifest list sha256:8979cf17b5a929255f570efe837792395db7  0.0s => => naming to docker.io/library/api:1.0                                  0.0s => => unpacking to docker.io/library/api:1.0                               0.0s~/api $ docker run --rm api:1.0 pwd/app

Your laptop`~/api`

No open hatches

Lunchboxes (containers)

No containers yet

Frozen packs (images)

node:24-alpine4 layers · 244MB

WORKDIR /app is your work counter. Every later line works there. RUN cooks while baking: here it installs packages. pwd prints the counter: /app.

Try it

Some lines bake a layer. Others only stick a note on the pack. Sort them.

Drag a card into a box. Or tap the card, then tap the box.

Try it

The image is built. Run it with hatch 3000 open, then knock with curl.

Your laptop`~/api`

No open hatches

Lunchboxes (containers)

No containers yet

Frozen packs (images)

api:1.08 layers · 244MB
node:24-alpine4 layers · 244MB

Hint

-p 3000:3000 opens the hatch: laptop port, then container port.

Predict

The recipe says EXPOSE 3000. You run the image with no -p. Can your laptop reach port 3000?

Check

~/api $ docker build -t api:1.0 .[+] Building 1.3s (10/10) FINISHED                                docker:default => [internal] load build definition from Dockerfile                        0.0s => => transferring dockerfile: 200B                                        0.0s => [internal] load metadata for docker.io/library/node:24-alpine           0.0s => [internal] load .dockerignore                                           0.0s => => transferring context: 2B                                             0.0s => [1/5] FROM docker.io/library/node:24-alpine@sha256:8a32c8cf649a49915a6  0.0s => => resolve docker.io/library/node:24-alpine@sha256:8a32c8cf649a49915a6  0.0s => [2/5] WORKDIR /app                                                      0.0s => [internal] load build context                                           0.0s => => transferring context: 675B                                           0.0s => [3/5] COPY package.json package-lock.json ./                            0.0s => [4/5] RUN npm ci --omit=dev                                             1.2s => [5/5] COPY server.js .                                                  0.0s => exporting to image                                                      0.1s => => exporting layers                                                     0.0s => => exporting manifest sha256:dc7800314dac8d66f0fd744a66287432eab0b43b6  0.0s => => exporting config sha256:00e8092d135c2dc5c2f015b20bb84dce74869227e7d  0.0s => => exporting attestation manifest sha256:9c156a337bb43693fe0cc0122bbdc  0.0s => => exporting manifest list sha256:6e4d6a6a470157a9a2bcaa8ec14418cc8756  0.0s => => naming to docker.io/library/api:1.0                                  0.0s => => unpacking to docker.io/library/api:1.0                               0.0s~/api $ docker run -d --name api api:1.09ae7e68b27cc6db02c55bc670db9280e69420649f964a99d83abb26f458d10b0~/api $ curl localhost:3000curl: (7) Failed to connect to localhost port 3000 after 0 ms: Couldn't connect to server~/api $ docker psCONTAINER ID   IMAGE     COMMAND                  CREATED         STATUS         PORTS      NAMES9ae7e68b27cc   api:1.0   "docker-entrypoint.s…"   6 seconds ago   Up 6 seconds   3000/tcp   api

Your laptop`~/api`

No open hatches

Lunchboxes (containers)

No containers yet

Frozen packs (images)

node:24-alpine4 layers · 244MB

Real error: Couldn't connect to server. Plain words: no hatch is open. docker ps shows 3000/tcp with no arrow.

Try it

ENV PORT=3000 is only a default. Build the command that runs the API on port 4000.

Tap the pieces in the right order.

…

You did it!

You can now use WORKDIR, RUN, ENV and EXPOSE.

Go deeper (optional)

Chain related commands in one RUN with &&. One RUN is one layer. EXPOSE also helps docker run -P, which opens every exposed port on a random laptop port.

Real mission

Add WORKDIR /app to your recipe from lesson 1.
Build it again.
docker build -t hello:1.1 .
Check the work folder.
docker run --rm hello:1.1 pwd

0 of 3 steps done · Your ticks are saved.

WORKDIR, RUN, ENV and EXPOSE

Your laptop~/api

Lunchboxes (containers)

Frozen packs (images)

Your laptop~/api

Lunchboxes (containers)

Frozen packs (images)

Your laptop~/api

Lunchboxes (containers)

Frozen packs (images)

Your laptop`~/api`

Your laptop`~/api`

Your laptop`~/api`