fix: remove quoted strings (hadolint)

Dockerfile

@@ -1,18 +1,18 @@
-FROM "ubuntu:18.04"
+FROM ubuntu:18.04
 LABEL maintainer="Julien Cabillot <dockerimages@cabillot.eu>"
 
-ENV DEBIAN_FRONTEND="noninteractive"
+ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get -qq update && \
     apt-get --no-install-recommends -qqy install siege && \
-    apt-get -qqy autoremove --purge  && \
+    apt-get -qqy autoremove --purge && \
     apt-get -qq clean && \
-    rm -rf "/usr/share/doc/*" \                                                                                        
+    rm -rf /usr/share/doc/* \
-       "/var/cache/*" \
+           /var/cache/* \
-       "/var/lib/apt/lists/*" \                                                                                        
+           /var/lib/apt/lists/* \
-       "/usr/src/*" \
+           /usr/src/* \
-       "/var/log/"{apt/*,dpkg.log} \                                                                                                                                                                                                          
+           /var/log/{apt/*,dpkg.log} \
-       "/tmp/*"
+           /tmp/*
 
 ENTRYPOINT [ "/usr/bin/siege" ]
 CMD [ "--help" ]