When building Docker images with Debian/Ubuntu packages, you might encounter this frustrating error:

debconf: unable to initialize frontend: Noninteractive
debconf: (Bareword "Debconf::FrontEnd::Noninteractive" not allowed while "strict subs" in use at (eval 35) line 3, <> line 1.)
debconf: falling back to frontend: Noninteractive
Subroutine BEGIN redefined at (eval 36) line 2, <> line 1.

This occurs when packages try to run interactive configuration during apt-get install, despite having set DEBIAN_FRONTEND=noninteractive. The error suggests Perl's strict mode is preventing debconf from properly initializing.

Solution 1: Combined ENV approach

ENV DEBIAN_FRONTEND=noninteractive \
    DEBCONF_NONINTERACTIVE_SEEN=true \
    DEBCONF_NOWARNINGS=true

RUN apt-get update && \
    apt-get install -y --no-install-recommends your-package

Solution 2: Preseeding package configuration

RUN echo "package package/question select answer" | debconf-set-selections && \
    echo "package package/another-question string value" | debconf-set-selections && \
    apt-get update && \
    apt-get install -y your-package

Solution 3: Docker build arguments

ARG DEBIAN_FRONTEND=noninteractive
RUN apt-get update && \
    apt-get install -y your-package

• Mixing ENV and RUN statements incorrectly
• Forgetting to chain apt-get update with install commands
• Not cleaning up apt cache in the same layer

FROM ubuntu:22.04

# Set all required debconf variables
ENV DEBIAN_FRONTEND=noninteractive \
    DEBCONF_NONINTERACTIVE_SEEN=true \
    DEBCONF_NOWARNINGS=true

# Install packages with proper cleanup
RUN apt-get update && \
    apt-get install -y --no-install-recommends \
        tzdata \
        postfix && \
    rm -rf /var/lib/apt/lists/*

When building Docker images based on Debian/Ubuntu, many developers encounter this frustrating message:

debconf: unable to initialize frontend: Noninteractive
debconf: (Bareword "Debconf::FrontEnd::Noninteractive" not allowed while "strict subs" in use)
debconf: falling back to frontend: Noninteractive

This typically appears even when you've explicitly set:

ENV DEBIAN_FRONTEND noninteractive

The issue stems from how debconf (Debian's configuration management system) handles environment variables during package installation. Some packages forcibly reset the frontend during their post-install scripts.

1. The Complete Environment Setup

Add these lines at the beginning of your Dockerfile:

ARG DEBIAN_FRONTEND=noninteractive
ENV DEBIAN_FRONTEND=$DEBIAN_FRONTEND
RUN echo 'debconf debconf/frontend select Noninteractive' | debconf-set-selections

2. For Problematic Packages

For packages that still cause issues (like mysql-server or postfix), use this pattern:

RUN apt-get update && \
    apt-get install -y --no-install-recommends \
    -o Dpkg::Options::="--force-confdef" \
    -o Dpkg::Options::="--force-confold" \
    your-package-here

3. The Nuclear Option

If nothing else works, override the problematic scripts:

RUN echo '#!/bin/sh' > /usr/sbin/policy-rc.d && \
    echo 'exit 101' >> /usr/sbin/policy-rc.d && \
    chmod +x /usr/sbin/policy-rc.d

Here's a complete working example for installing MySQL in Docker:

FROM ubuntu:22.04

ARG DEBIAN_FRONTEND=noninteractive
ENV DEBIAN_FRONTEND=$DEBIAN_FRONTEND

RUN echo 'debconf debconf/frontend select Noninteractive' | debconf-set-selections && \
    apt-get update && \
    apt-get install -y --no-install-recommends \
    -o Dpkg::Options::="--force-confdef" \
    -o Dpkg::Options::="--force-confold" \
    mysql-server && \
    rm -rf /var/lib/apt/lists/*

• The --no-install-recommends flag helps reduce image size
• Always clean apt cache with rm -rf /var/lib/apt/lists/*
• Consider using multi-stage builds for production images