Avoid root when prefix is writable

Previously if the prefix was non-existent we would switch to root even
if the user does have the permissions to create the directory.

Fixes #3585

.github/workflows/installer.yml

@@ -24,6 +24,24 @@ jobs:
       - name: Test code-server
         run: yarn test:standalone-release code-server
 
+  alpine:
+    name: Test installer on Alpine
+    runs-on: ubuntu-latest
+    container: "alpine:3.14"
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v2
+
+      - name: Install curl
+        run: apk add curl
+
+      - name: Add user
+        run: adduser coder --disabled-password
+
+      # Standalone should work without root.
+      - name: Test standalone to a non-existent prefix
+        run: su coder -c "./install.sh --method standalone --prefix /tmp/does/not/yet/exist"
+
   macos:
     name: Test installer on macOS
     runs-on: macos-latest

install.sh

@@ -132,7 +132,6 @@ main() {
   unset \
     DRY_RUN \
     METHOD \
-    STANDALONE_INSTALL_PREFIX \
     OPTIONAL \
     ALL_FLAGS \
     RSH_ARGS \
@@ -381,6 +380,10 @@ install_standalone() {
   fetch "https://github.com/cdr/code-server/releases/download/v$VERSION/code-server-$VERSION-$OS-$ARCH.tar.gz" \
     "$CACHE_DIR/code-server-$VERSION-$OS-$ARCH.tar.gz"
 
+  # -w only works if the directory exists so try creating it first. If this
+  # fails we can ignore the error as the -w check will then swap us to sudo.
+  sh_c mkdir -p "$STANDALONE_INSTALL_PREFIX" 2> /dev/null || true
+
   sh_c="sh_c"
   if [ ! -w "$STANDALONE_INSTALL_PREFIX" ]; then
     sh_c="sudo_sh_c"