The Same Build, Run Twice

Build once, change nothing, build again, and the output tells you exactly what it did.

bash

$ docker build -t my-api:1 .
 => [2/5] WORKDIR /app                      0.1s
 => [3/5] COPY package.json package-lock.json ./   0.0s
 => [4/5] RUN npm ci                        9.8s
 => [5/5] COPY . .                          0.1s
 => exporting to image                      1.2s

bash

$ docker build -t my-api:1 .
 => CACHED [2/5] WORKDIR /app
 => CACHED [3/5] COPY package.json package-lock.json ./
 => CACHED [4/5] RUN npm ci
 => CACHED [5/5] COPY . .
 => exporting to image                      0.3s

CACHED on every step. The nine seconds of npm ci were not spent again, because the instruction and the layer underneath it were both unchanged.

Docker
Containers
CI/CD