Optimize Docker stage caching by moving app copy after package installations

[Copilot]

The Docker stages were copying application source code before installing packages, which caused unnecessary cache invalidation. When app files changed, it would force rebuilding expensive package installation layers even though the dependencies hadn't changed.

Problem

FROM base AS debug
COPY . /app/src          # ❌ App changes invalidate package cache
RUN apt-get install...   # ❌ Rebuilds when app changes

FROM base AS tests  
COPY . /app/src              # ❌ App changes invalidate package cache
RUN apt-get install...      # ❌ Rebuilds when app changes
RUN npx playwright install  # ❌ Expensive operation rebuilds unnecessarily

Solution

FROM base AS debug
RUN apt-get install...   # ✅ Cached independently of app changes
COPY . /app/src          # ✅ Only invalidates when needed

FROM base AS tests
RUN apt-get install...      # ✅ Cached independently of app changes  
RUN npx playwright install  # ✅ Expensive operation stays cached
COPY . /app/src             # ✅ Only invalidates when needed

Impact

• Improved build performance: Package installations now cache properly across app changes
• Reduced CI/CD build times: Expensive operations like Playwright installation only run when dependencies change
• Better layer caching: Each Docker layer caches independently based on its actual dependencies
• Faster development iteration: App code changes don't trigger unnecessary package reinstalls

The changes move COPY . /app/src commands to the end of each stage, after all package installations are complete. This ensures optimal Docker layer caching behavior.

Fixes #245.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[dannystaple]

I left a typo in the original ticket (typed on a mobile). IT should read "base docker stage" and has nothing to do with sockets.

[github-actions]

🦙 MegaLinter status: ❌ ERROR

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
❌ ACTION	actionlint	5		6	0	0.09s
✅ BASH	bash-exec	1		0	0	0.0s
✅ BASH	shellcheck	1		0	0	0.05s
⚠️ BASH	shfmt	1		1	0	0.0s
❌ C	cppcheck	8		8	0	0.23s
❌ CPP	cppcheck	8		8	0	0.33s
✅ JSON	npm-package-json-lint	yes		no	no	0.48s
✅ JSON	v8r	12		0	0	9.34s
❌ MARKDOWN	markdown-link-check	736		213	0	38.45s
✅ REPOSITORY	gitleaks	yes		no	no	14.14s
✅ REPOSITORY	git_diff	yes		no	no	0.69s
✅ REPOSITORY	grype	yes		no	no	34.36s
✅ REPOSITORY	secretlint	yes		no	no	44.79s
✅ REPOSITORY	syft	yes		no	no	1.97s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.19s
✅ REPOSITORY	trufflehog	yes		no	no	4.72s
✅ XML	xmllint	1		0	0	189.22s
✅ YAML	v8r	13		0	0	7.84s

See detailed report in MegaLinter reports