build: follow-up prev: only use host userid for local dev builds

reproducibility probably needs a hardcoded userid

Also, move the UID arg later in the dockerfiles, for better caching.
(if local dev build and repro build set different UIDs, the build caches
will diverge at that step)

contrib/build-linux/appimage/Dockerfile

@@ -4,8 +4,6 @@
 
 FROM debian:buster@sha256:233c3bbc892229c82da7231980d50adceba4db56a08c0b7053a4852782703459
 
-ARG UID=1000
-
 ENV LC_ALL=C.UTF-8 LANG=C.UTF-8
 ENV DEBIAN_FRONTEND=noninteractive
 
@@ -73,6 +71,7 @@ RUN apt-get update -q && \
     apt-get clean
 
 # create new user to avoid using root; but with sudo access and no password for convenience.
+ARG UID=1000
 ENV USER="user"
 ENV HOME_DIR="/home/${USER}"
 ENV WORK_DIR="${HOME_DIR}/wspace" \

contrib/build-linux/appimage/build.sh

@@ -22,10 +22,13 @@ if [ ! -z "$ELECBUILD_NOCACHE" ] ; then
     DOCKER_BUILD_FLAGS="--pull --no-cache"
 fi
 
+if [ -z "$ELECBUILD_COMMIT" ] ; then  # local dev build
+    DOCKER_BUILD_FLAGS="$DOCKER_BUILD_FLAGS --build-arg UID=$BUILD_UID"
+fi
+
 info "building docker image."
 docker build \
     $DOCKER_BUILD_FLAGS \
-    --build-arg UID=$BUILD_UID \
     -t electrum-appimage-builder-img \
     "$CONTRIB_APPIMAGE"