fix(ssh): survive read-only ~/.ssh ControlPath; render sidecar on all host OSes

Coordinated with the pi-extensions ssh-controlmaster fix (picked up at build via
PI_EXTENSIONS_REF=main), this makes `pi --ssh <host>` and `dssh`/`dscp` robust
to a user ~/.ssh/config whose per-host ControlPath points under the read-only
~/.ssh bind-mount (e.g. `ControlPath ~/.ssh/cm/%r@%h:%p`). A system default can
never override a user's per-host value, so the fix lives in two layers.

- setup-lan-access.sh: always render the writable ~/.ssh-local/config sidecar
  (Host * ControlPath redirect into ~/.ssh-local/cm + Include ~/.ssh/config) on
  EVERY host OS. Previously the script exited early (no-op) on native Linux,
  leaving dssh/dscp broken when ~/.ssh was read-only there too. The host-jump
  block, its key generation, and the authorize hints stay gated on VM-backed
  detection / DEVBOX_LAN_ACCESS=jump (new NEED_JUMP flag).
- Dockerfile.base: document that the /etc/ssh drop-in default cannot override a
  user per-host ControlPath; cross-ref the two handling layers.
- entrypoint-user.sh: correct the now-stale "no-op on native Linux" comment.
- README.md / DOCKER_HUB.md: document read-only-~/.ssh ControlPath handling.

CHANGELOG: v1.1.5 (Fixed + Changed + pi 0.79.6 -> 0.79.7 auto-resolved bump).

rootfs/usr/local/lib/pi-devbox/setup-lan-access.sh

@@ -14,7 +14,9 @@
 #   The one thing reachable from a container on every OS is the host itself
 #   (host.docker.internal). So on VM-backed hosts we generate a writable SSH
 #   config that reaches the host and lets the user ProxyJump onward to LAN
-#   peers the host can reach. On native Linux we do nothing.
+#   peers the host can reach. On native Linux we render the same writable
+#   config (for the ControlPath redirect + Include ~/.ssh/config) but emit no
+#   jump block, since LAN peers are reachable directly there.
 #
 #   We ship the MECHANISM (a generic `host` jump alias + writable config),
 #   never the POLICY: the user's specific target hosts live in their own
@@ -30,7 +32,9 @@
 #
 # CONTROLS (env)
 #   DEVBOX_LAN_ACCESS = auto (default) | jump | off
-#       auto  → set up the jump config only on VM-backed hosts; no-op on Linux.
+#       auto  → set up the host jump only on VM-backed hosts. The writable
+#               sidecar config (ControlPath redirect + Include) is always
+#               rendered, on every OS.
 #       jump  → always set up (e.g. native Linux with extra_hosts host-gateway).
 #       off   → do nothing.
 #   HOST_SSH_USER  — the username to SSH into the host as. REQUIRED for the
@@ -84,42 +88,72 @@ is_vm_backed() {
   getent hosts "$HOST_ALIAS_HOSTNAME" >/dev/null 2>&1
 }
 
-if [ "$MODE" = "auto" ] && ! is_vm_backed; then
-  # Native Linux host: LAN peers are reachable directly. Nothing to do.
-  exit 0
-fi
-
-# From here: MODE=jump, or MODE=auto on a VM-backed host.
-
-command -v ssh-keygen >/dev/null 2>&1 || exit 0
-
+# ── Writable socket dir + sidecar (ALWAYS, every host OS) ─────────────
+# The ControlPath redirect in the generated config needs a writable directory
+# regardless of host OS or jump mode. ~/.ssh is typically read-only, so the
+# master socket lives under the writable ~/.ssh-local. We create it and render
+# the config UNCONDITIONALLY so the redirect (and `Include ~/.ssh/config`) works
+# even on native Linux — where we set up no host jump but a read-only ~/.ssh
+# would otherwise still break ControlMaster sockets.
 mkdir -p "${SSH_LOCAL}/cm" 2>/dev/null || true
 chmod 700 "${SSH_LOCAL}" "${SSH_LOCAL}/cm" 2>/dev/null || true
 
-# ── Jump key (generated once; preserved across restarts) ──────────────
+# ── Decide whether to set up the host jump ────────────────────────────
+# Jump = reach the container host (host.docker.internal) as an SSH ProxyJump
+# onward to the host's LAN peers. Needed on VM-backed hosts (macOS / Docker
+# Desktop) or when forced with DEVBOX_LAN_ACCESS=jump. On native Linux LAN
+# peers are reachable directly, so NEED_JUMP=0 and we emit no jump block — but
+# we still render the config for the ControlPath redirect + Include.
+NEED_JUMP=0
+if [ "$MODE" = "jump" ] || { [ "$MODE" = "auto" ] && is_vm_backed; }; then
+  NEED_JUMP=1
+fi
+
+# ── Jump key (only when a jump is needed; generated once, preserved) ──
 # Persisted via a named volume on ~/.ssh-local (see compose), so a fresh key
 # is generated only on the very first start (or if the volume is wiped). When
 # we DO generate one it must be (re-)authorized on the host, so we flag it and
 # print a copy-paste authorize line below.
 KEY_JUST_GENERATED=0
-if [ ! -f "$KEY" ]; then
-  ssh-keygen -t ed25519 -N '' -C "devbox-jump@${HOSTNAME:-container}" -f "$KEY" >/dev/null 2>&1 || exit 0
-  chmod 600 "$KEY" 2>/dev/null || true
-  KEY_JUST_GENERATED=1
+if [ "$NEED_JUMP" = "1" ] && command -v ssh-keygen >/dev/null 2>&1 && [ ! -f "$KEY" ]; then
+  if ssh-keygen -t ed25519 -N '' -C "devbox-jump@${HOSTNAME:-container}" -f "$KEY" >/dev/null 2>&1; then
+    chmod 600 "$KEY" 2>/dev/null || true
+    KEY_JUST_GENERATED=1
+  fi
 fi
 
 # ── Render the writable config ────────────────────────────────────────
-USER_LINE=""
-if [ -n "${HOST_SSH_USER:-}" ]; then
-  USER_LINE="    User ${HOST_SSH_USER}"
-fi
-
-# Optional host-owned named-peer jump overrides (portable: lives on the host,
-# not in the image). Included BEFORE ~/.ssh/config so its ProxyJump wins.
-SSH_LAN_CONF="${HOME}/.config/devbox-shell/ssh-lan.conf"
+# Jump-specific blocks (the host alias, host-owned peer overrides, and the
+# optional RFC1918 catch-all) only make sense when a jump is set up; on native
+# Linux they are all empty and only the ControlPath redirect + Include remain.
+JUMP_BLOCK=""
 LAN_CONF_BLOCK=""
-if [ -r "$SSH_LAN_CONF" ]; then
-  LAN_CONF_BLOCK=$(cat <<'EOF'
+AUTOJUMP_BLOCK=""
+if [ "$NEED_JUMP" = "1" ]; then
+  USER_LINE=""
+  if [ -n "${HOST_SSH_USER:-}" ]; then
+    USER_LINE="    User ${HOST_SSH_USER}"
+  fi
+  JUMP_BLOCK=$(cat <<EOF
+
+# The container host (OrbStack / Docker Desktop). 'host' and 'mac' are aliases.
+Host host mac
+    HostName ${HOST_ALIAS_HOSTNAME}
+${USER_LINE}
+    IdentityFile ~/.ssh-local/devbox_jump_ed25519
+    IdentitiesOnly yes
+    ControlMaster auto
+    ControlPath ~/.ssh-local/cm/%r@%h:%p
+    ControlPersist 4h
+    ServerAliveInterval 30
+EOF
+)
+
+  # Optional host-owned named-peer jump overrides (portable: lives on the host,
+  # not in the image). Included BEFORE ~/.ssh/config so its ProxyJump wins.
+  SSH_LAN_CONF="${HOME}/.config/devbox-shell/ssh-lan.conf"
+  if [ -r "$SSH_LAN_CONF" ]; then
+    LAN_CONF_BLOCK=$(cat <<'EOF'
 
 # Host-owned named-peer jump overrides (bind-mounted; edit on the host).
 # Scope reset to match-all so the Include applies to every target host.
@@ -127,14 +161,13 @@ Host *
 Include ~/.config/devbox-shell/ssh-lan.conf
 EOF
 )
-fi
+  fi
 
-# Optional opt-in RFC1918 catch-all: ProxyJump every private IP through the
-# host. Matches the typed address, never the resolved HostName, so named hosts
-# with their own ProxyJump are unaffected. Network-agnostic → roaming-safe.
-AUTOJUMP_BLOCK=""
-if [ "${DEVBOX_LAN_AUTOJUMP_PRIVATE:-0}" = "1" ]; then
-  AUTOJUMP_BLOCK=$(cat <<'EOF'
+  # Optional opt-in RFC1918 catch-all: ProxyJump every private IP through the
+  # host. Matches the typed address, never the resolved HostName, so named hosts
+  # with their own ProxyJump are unaffected. Network-agnostic → roaming-safe.
+  if [ "${DEVBOX_LAN_AUTOJUMP_PRIVATE:-0}" = "1" ]; then
+    AUTOJUMP_BLOCK=$(cat <<'EOF'
 
 # RFC1918 auto-jump (DEVBOX_LAN_AUTOJUMP_PRIVATE=1): reach any private IP on
 # the host's CURRENT LAN via bare `dssh user@<ip>`. Public IPs are unmatched
@@ -146,6 +179,7 @@ Host 10.* 192.168.* 172.16.* 172.17.* 172.18.* 172.19.* 172.20.* 172.21.* 172.22
     ProxyJump host
 EOF
 )
+  fi
 fi
 
 INCLUDE_BLOCK=""
@@ -176,17 +210,7 @@ Host *
     UserKnownHostsFile ~/.ssh-local/known_hosts
     StrictHostKeyChecking accept-new
     ControlPath ~/.ssh-local/cm/%r@%h:%p
-
-# The container host (OrbStack / Docker Desktop). 'host' and 'mac' are aliases.
-Host host mac
-    HostName ${HOST_ALIAS_HOSTNAME}
-${USER_LINE}
-    IdentityFile ~/.ssh-local/devbox_jump_ed25519
-    IdentitiesOnly yes
-    ControlMaster auto
-    ControlPath ~/.ssh-local/cm/%r@%h:%p
-    ControlPersist 4h
-    ServerAliveInterval 30
+${JUMP_BLOCK}
 ${LAN_CONF_BLOCK}
 ${AUTOJUMP_BLOCK}
 ${INCLUDE_BLOCK}
@@ -199,6 +223,7 @@ chmod 600 "$CONFIG" 2>/dev/null || true
 # host won't recognize. With ~/.ssh-local persisted via a named volume, case
 # (b) fires only on first-ever start (or after the volume is reset) — so this
 # is normally a one-time, one-line step per machine, with no file to locate.
+if [ "$NEED_JUMP" = "1" ]; then
 PUBKEY_TEXT="$(cat "${KEY}.pub" 2>/dev/null)"
 if [ -z "${HOST_SSH_USER:-}" ]; then
   cat <<EOF
@@ -221,5 +246,6 @@ elif [ "$KEY_JUST_GENERATED" = "1" ]; then
          repeat this on container updates — only if that volume is reset.
 EOF
 fi
+fi
 
 exit 0