Quote further variables

Dockerfile

@@ -21,7 +21,7 @@ RUN \
         vim \
         xmlstarlet \
     && {  \
-            [ $DEBUG -eq 1 ] \
+            [ $"DEBUG" -eq 1 ] \
             && apt-get install -y \
                 build-essential \
                 imagemagick \
@@ -32,7 +32,7 @@ RUN \
 RUN pip install pyocd
 
 RUN \
-    [ $DEBUG -eq 1 ] && { \
+    [ "$DEBUG" -eq 1 ] && { \
         git clone https://github.com/stefanhaustein/TerminalImageViewer.git && \
         cd TerminalImageViewer/src/main/cpp && \
         make && \
@@ -43,7 +43,7 @@ RUN \
     || true
 
 RUN \
-    wget ${KEIL_URL} --progress=bar:force:noscroll -O MDK537.exe; \
+    wget "${KEIL_URL}" --progress=bar:force:noscroll -O MDK537.exe; \
     [ $DEBUG -eq 1 ] && { \
         mkdir -p logs; \
         mkdir -p screens; \
@@ -147,7 +147,7 @@ RUN \
     { \
         # Improve arguments for screenshots
         ARGS="$([ $DEBUG -eq 1 ] && echo "-screen 0 640x480x24 -dpi 192" || echo "")"; \
-        Xvfb ${DISPLAY} ${ARGS} & \
+        Xvfb "${DISPLAY}" ${ARGS} & \
     }; \
     displayPID="$!"; \
     # Wait for Xvfb to start