What is multi-stage build in Docker and why is it needed?

Multi-stage build in Docker is a technique that allows using multiple build stages within a single Dockerfile. The main idea is to split the build process into several steps, where the initial stages involve compiling or preparing artifacts, and the final stage creates a minimal image containing only the files necessary for running.

Why is this needed:

• Reducing the size of the final image, as build tools and temporary files are not included.
• Increasing security, since the final image does not contain unnecessary utilities and source code.
• Ease of maintenance and readability of the Dockerfile, as all stages are described in one place.

Example:

# Build stage
FROM golang:1.20 AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp

# Final minimal image
FROM alpine:latest
WORKDIR /app
COPY --from=builder /app/myapp .
CMD ["./myapp"]

Here, the first stage compiles the binary, and the second creates a lightweight image containing only this binary.