feat(dock_from_renv): default to non-root user 'rstudio' out of the box

[VincentGuyader]

Summary

Closes #100.

Running the container as root is bad practice; the package's own
default FROM = "rocker/r-base" ships an rstudio user precisely
to make non-root operation easy. Up to now dock_from_renv()
required the caller to pass user = "rstudio" explicitly, and even
then the cache mount target was wrong (left at /root/.cache/R/renv
which rstudio cannot write to).

This PR makes user = "rstudio" the default and ensures the
generated Dockerfile actually works for any non-root user passed.

Three changes, applied as a unit

1. Default user flips from NULL to "rstudio"

The runtime container now drops privilege to rstudio by default.
Pass user = NULL to opt out and keep the previous root behaviour.

2. Defensive useradd at the top of the Dockerfile

RUN id -u <user> >/dev/null 2>&1 || useradd -m -d /home/<user> -s /bin/bash <user>

Idempotent: a no-op on rocker/* images where the user already
exists, a real useradd on r-base, ubuntu:*, debian:*. Works
for any user (rstudio, kevin, myapp, ...) on debian/ubuntu.

For alpine-based images, pass user = NULL and create the user
yourself (alpine uses adduser, not useradd).

3. Auto-derive renv_paths_cache and chown it before USER

renv_paths_cache is auto-derived from user when not supplied:

• user = NULL -> /root/.cache/R/renv (legacy).
• user = "rstudio" -> /home/rstudio/.cache/R/renv (new default).
• user = "kevin" -> /home/kevin/.cache/R/renv.

The Dockerfile emits an explicit
RUN mkdir -p && chown -R <user>:<user> ${RENV_PATHS_CACHE}
step right before the USER <user> directive, so the
--mount=type=cache,target=${RENV_PATHS_CACHE} later is writable.

The USER directive is moved from immediately after the ARG/ENV
setup (where it broke apt-get because apt-get needs root) to
right before the renv::restore() cache-mount RUN. All install
steps run as root; only renv::restore() and the runtime container
run as <user>.

Behaviour change

For users regenerating their Dockerfile without specifying user:

+RUN id -u rstudio >/dev/null 2>&1 || useradd -m -d /home/rstudio ...
-ARG RENV_PATHS_CACHE=/root/.cache/R/renv
+ARG RENV_PATHS_CACHE=/home/rstudio/.cache/R/renv
 ...
+RUN mkdir -p ${RENV_PATHS_CACHE}
+RUN chown -R rstudio:rstudio ${RENV_PATHS_CACHE}
+USER rstudio
 RUN --mount=type=cache,...,target=${RENV_PATHS_CACHE} R -e 'renv::restore()'

Test plan

• TDD red-first: 5 new tests cover all branches.
  • user = NULL: no USER, no chown, no useradd, cache at /root.
  • default (user = "rstudio"): cache at /home/rstudio.
  • ordering invariant: apt-get -> chown -> USER -> renv::restore.
  • explicit renv_paths_cache + user: explicit path honoured, chown applied.
  • defensive useradd at the top of the Dockerfile.
• Existing test dock_from_renv works updated; fixture
  tests/testthat/renv_Dockerfile regenerated.
• devtools::test() -> [ FAIL 0 | WARN 3 | SKIP 0 | PASS 253 ].
• R CMD check --as-cran -> 0 errors / 0 warnings / 0 notes
  (2m 45s).

Context

Tier A item from the post-CRAN-prep roadmap. Companion to #105
(strict_install for dock_from_desc).

[Copilot]

Pull request overview

This PR updates dock_from_renv() to generate Dockerfiles that run as a non-root user by default (now user = "rstudio"), fixing the prior mismatch where a non-root USER could be combined with a root-owned RENV_PATHS_CACHE. It also adjusts Dockerfile ordering so root-required steps run before dropping privileges, and expands tests/docs accordingly.

Changes:

• Default user changes from NULL to "rstudio", with user = NULL preserving legacy root behavior.
• renv_paths_cache now defaults to NULL and is auto-derived from user, with a pre-USER mkdir/chown to ensure the cache mount target is writable.
• Adds a defensive useradd (guarded by id -u) and updates tests, fixtures, NEWS, and generated Rd docs.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
R/dock_from_renv.R	Implements new defaults, cache path derivation, defensive user creation, and moves privilege drop to just before renv::restore().
tests/testthat/test-dock_from_renv.R	Adds/updates tests to cover new user/cache behavior and Dockerfile ordering invariants.
tests/testthat/renv_Dockerfile	Regenerates the expected Dockerfile fixture to reflect new instructions/order.
man/dock_from_renv.Rd	Updates generated documentation for new parameter defaults and behavior.
NEWS.md	Documents the behavior change and opt-out for legacy root behavior.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.