fix: stop shipping a second copy of the toolchain in every Railpack image

[jeroenrinzema]

Every Railpack image we build ships a byte-identical second copy of the toolchain. A stock create-next-app is 1163 MB on disk; 194 MB of that is the duplicate.

Cause

The non-root layer ended with:

&& if [ -d /mise ]; then chown -R 1000:1000 /mise; fi

/mise arrives from an earlier layer. chown rewrites every file it touches, so BuildKit records the entire tree again in the layer users pull — same bytes, different owner. It is the single largest layer we add and it contributes nothing.

	compressed	our non-root layer
before	357.5 MB	58.7 MB
after	298.8 MB	0.0 MB

Why the chown was unnecessary

/mise/installs is already drwxr-xr-x root:root, so uid 1000 has the read and execute it actually uses.

What mise does need is somewhere to write. Drop the chown outright and it prints [WARN] migrate: failed create_dir_all: /mise/migrations on every start — non-fatal, but it lands in the user's logs. It wants exactly two directories, and MISE_CACHE_DIR=/mise/cache is in the runtime env while that directory does not exist at all. Creating both empty costs nothing measurable and leaves no warning.

Verification

Built through the same path the CLI drives (Railpack v0.35.0, linux/arm64, same frontend and plan), on two stacks so this is not a Node-only claim:

stack	before	after
create-next-app (Next 16.3.0)	1163 MB	969 MB
Flask + gunicorn (Python 3.13)	407 MB	328 MB

For each, as uid=1000(railpack):

• app serves HTTP 200 (Next.js also serves /_next/static/…css)
• node/npm and python/pip resolve through /mise/shims
• mise exec -- … and mise ls succeed
• no permission-denied or create_dir_all warning remains

The layout.runtime_user == 1000:1000 assertion in build() still holds — USER 1000:1000 is unchanged, so the guard against a root image is untouched.

Not addressed here

cp -a /root/. /home/railpack/ leaves two copies of whatever Railpack puts in /root (it includes /root/.cache in the deploy step). That was ~7 MB on both stacks, and unlike the chown the copy is doing real work — the runtime user's HOME moves — so it needs a different fix than deleting a line.

Separately: Railpack v0.35.0 ignores Next.js output: 'standalone' — the generated deploy plan is byte-identical with and without it, and /app/node_modules ships unconditionally. Shrinking that is a railpack.json matter for the app, not a CLI change.