fix(peer-cli): use writable Docker config for image builds

The peer CLI Docker recipes failed on this host because Docker Buildx tried to
write activity metadata under /home/pfs/.docker, which is read-only here. Use a
repo-local ignored Docker config by default, while still respecting an explicit
DOCKER_CONFIG from the caller.

This keeps the peer-cli image and network recipes runnable without requiring
host-global Docker client state to be writable.

Test Plan:
- just peer-cli-image

Refs: PEER_CLI_SCENARIOS.md

justfile

@@ -1,5 +1,6 @@
 export RUSTFLAGS := "-C target-cpu=native"
 export WEBKIT_DISABLE_COMPOSITING_MODE := "1"
+export DOCKER_CONFIG := env_var_or_default("DOCKER_CONFIG", ".lanspread-peer-cli/docker-config")
 
 build:
   cargo tauri build --no-bundle -- --profile dev
@@ -29,6 +30,7 @@ peer-cli-build:
   cargo build -p lanspread-peer-cli
 
 peer-cli-image:
+  mkdir -p "$DOCKER_CONFIG"
   docker build -f crates/lanspread-peer-cli/Dockerfile -t lanspread-peer-cli:dev .
 
 # macvlan: each peer container gets its own MAC/IP on the real LAN.
@@ -40,6 +42,7 @@ LANSPREAD_SUBNET := "192.168.1.0/24"
 LANSPREAD_GATEWAY := "192.168.1.1"
 
 peer-cli-net:
+  mkdir -p "$DOCKER_CONFIG"
   docker network inspect {{LANSPREAD_NET}} >/dev/null 2>&1 || \
     docker network create -d macvlan \
       --subnet={{LANSPREAD_SUBNET}} \