Optimize Docker build with layer caching and .dockerignore

[Copilot]

Docker builds were slow due to poor layer caching (copying all code before deps), running migrations at build time, and no .dockerignore.

Changes

• Added .dockerignore — excludes .git/, screenshots/, terraform/, docs, and build artifacts (~1MB+ reduction in build context)

• Optimized Dockerfile:

  • Switch from alpine:latest to python:3.11-slim (specific version, smaller image)
  • Copy requirements.txt first, install deps, then copy code (enables dependency layer caching)
  • Move migrate to container startup via CMD (was baked into build)
  • Remove duplicate psycopg2 install (already in requirements.txt) and unused Pillow
  • Consolidate RUN commands and clean apt cache in same layer

# Before: deps reinstalled on every code change
COPY . /app
RUN pip install -r requirements.txt

# After: deps cached when only code changes
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .

Original prompt

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This section details on the original issue you should resolve

<issue_title>Painful Build</issue_title>
<issue_description>We are aware that the docker build process is ugly and slow and now looking at how to make this leaner and faster for all.

</issue_description>

<agent_instructions>Try and optimise the docker build process so it isn't painful and slow </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Painful Build #61

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