Build cache and image size

Key points

• A layer is reused only if its instruction and inputs are unchanged. One miss invalidates every layer after it.
• So order from least-changing to most-changing: base image, then dependencies, then your source code.
• BuildKit is the default builder since Docker 23. RUN --mount=type=cache keeps the Maven repo warm without baking it into a layer.
• Spring Boot 3.3+ can split a fat jar: java -Djarmode=tools -jar app.jar extract --layers gives dependencies their own layer.
• Clean up inside the same RUN that made the mess. A later rm hides bytes but still ships them.
• docker image history tells you which instruction cost the megabytes. Read it before you start guessing.

Example

FROM maven:3.9-eclipse-temurin-21 AS build
WORKDIR /src

# cache mount: ~/.m2 is reused between builds and never enters a layer
COPY pom.xml .
RUN --mount=type=cache,target=/root/.m2 mvn -B -q dependency:go-offline

COPY src ./src
RUN --mount=type=cache,target=/root/.m2 mvn -B -q package -DskipTests

# where did the size go, and what did the builder actually do?
#   docker image history myapp:1.0 --no-trunc
#   docker build --progress=plain -t myapp:1.0 .

Cheap, stable layers first. Your source code goes last.