How to Fix Docker Build Failure in Elixir

Running into a Docker Build Failure in Elixir? This guide walks you through the root cause and a practical fix.

Why This Happens

Docker build failures in Elixir usually come from missing system dependencies, incorrect base images, or build steps that require files not yet copied into the container. The minimal container environment differs significantly from your local machine.

How to Fix It

The key is to use alpine-based multi-stage build and copy mix files first for dependency caching:

FROM elixir:1.15-alpine AS build
RUN apk add --no-cache build-base
WORKDIR /app
COPY mix.exs mix.lock ./
RUN mix deps.get --only prod && mix deps.compile
COPY . .
ENV MIX_ENV=prod
RUN mix release

FROM alpine:3.18
COPY --from=build /app/_build/prod/rel/my_app ./
CMD ["bin/my_app", "start"]

Common Pitfall

One pitfall to avoid: applying a quick workaround that disables the underlying safety check. This masks the real problem and will come back to haunt you later. Consider adding a health check endpoint or startup validation that catches this misconfiguration before it reaches users.

Testing Your Changes

Run your test suite to make sure the fix doesn't introduce regressions. If you don't have tests covering this area, now is a good time to add a simple integration test. A quick manual smoke test across different browsers or environments can also catch edge cases your tests might miss.

Monitoring

Tools like [Bugsly](https://bugsly.dev) can catch these Elixir errors in real time, giving you stack traces and context to fix issues faster.