Master Dockerfile: Core Commands, Multi‑Stage Builds, and Handy Tricks

Dockerfile is a plain‑text script that defines how to build a Docker image by specifying each step and configuration.

Key Functions

1. Define a base image

FROM ubuntu:22.04</code><code># or any other base such as openjdk:11-jdk

2. Install dependencies using RUN to execute shell commands.

RUN apt-get update && apt-get install -y \
    python3 \
    pip \
    curl

3. Copy local files into the image with COPY.

COPY ./app /app</code><code>COPY requirements.txt /app/

4. Set the working directory with WORKDIR, which creates the directory if it does not exist. WORKDIR /app Relative paths can be chained, e.g.,

WORKDIR /app
WORKDIR src        # resolves to /app/src
WORKDIR ../data    # resolves to /app/data

5. Define environment variables using ENV.

ENV JAVA_HOME=/usr/lib/jvm/java-11-openjdk
ENV PATH=$PATH:$JAVA_HOME/bin

6. Expose ports with EXPOSE. EXPOSE 8080 7. Specify the start command using CMD (overridable) or ENTRYPOINT (fixed).

CMD ["python3", "app.py"]
# or
ENTRYPOINT ["java", "-jar", "app.jar"]

Both can be combined; for example, an Nginx image:

FROM nginx
ENTRYPOINT ["nginx", "-g"]
CMD ["daemon off;"]

Basic Dockerfile Example

# 1. Base image
FROM ubuntu:22.04

# 2. Maintainer (optional)
LABEL maintainer="[email protected]"

# 3. Environment variables
ENV DEBIAN_FRONTEND=noninteractive
ENV JAVA_HOME=/usr/lib/jvm/java-11-openjdk

# 4. Install dependencies
RUN apt-get update && \
    apt-get install -y \
    openjdk-11-jdk \
    maven && \
    rm -rf /var/lib/apt/lists/*

# 5. Set work directory
WORKDIR /app

# 6. Copy source files
COPY pom.xml .
COPY src ./src

# 7. Build the application
RUN mvn clean package -DskipTests

# 8. Expose port
EXPOSE 8080

# 9. Start the app
CMD ["java", "-jar", "target/app.jar"]

Multi‑Stage Build (Reduce Image Size)

# Build stage
FROM maven:3.9-eclipse-temurin-11 AS builder
WORKDIR /build
COPY . .
RUN mvn clean package

# Runtime stage (only runtime artifacts)
FROM eclipse-temurin:11-jre
WORKDIR /app
COPY --from=builder /build/target/app.jar .
CMD ["java", "-jar", "app.jar"]

Common Docker Commands

# Build the image
docker build -t my-app:1.0 .

# Run a container
docker run -p 8080:8080 my-app:1.0

# List images
docker images

# View build history
docker history my-app:1.0

Practical Trick

Append || true to rm commands to prevent build failures when the file does not exist.

# Without || true – build fails if file is missing
RUN rm /opt/spark/jars/arrow-c-data-15.0.0.jar
# Error: rm: cannot remove ...: No such file or directory

# With || true – command always succeeds
RUN rm /opt/spark/jars/arrow-c-data-15.0.0.jar || true

In summary, Dockerfile enables automated, reproducible image creation, ensures consistent environments, simplifies deployment, supports version control, and can be optimized with multi‑stage builds and small reliability tricks.