appmgr: only mount binds if installed

also improve build scripts

appmgr/build-portable.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+set -e
+shopt -s expand_aliases
+
+if [ "$0" != "./build-portable.sh" ]; then
+	>&2 echo "Must be run from appmgr directory"
+	exit 1
+fi
+
+alias 'rust-musl-builder'='docker run --rm -it -v "$HOME"/.cargo/registry:/root/.cargo/registry -v "$(pwd)":/home/rust/src messense/rust-musl-cross:x86_64-musl'
+
+cd ..
+rust-musl-builder sh -c "(cd appmgr && cargo build --release --target=x86_64-unknown-linux-musl --features=portable,production --no-default-features)"
+cd appmgr