fix: remove quoted strings (hadolint)

Dockerfile

@@ -1,19 +1,18 @@
-FROM "ubuntu:18.04"
+FROM ubuntu:18.04
 LABEL maintainer="Julien Cabillot <dockerimages@cabillot.eu>"
 
-RUN export DEBIAN_FRONTEND="noninteractive" && \
-    export RUNTIME_PACKAGES="sslscan" && \
+RUN export DEBIAN_FRONTEND=noninteractive && \
+    export RUNTIME_PACKAGES=sslscan && \
     apt-get -qq update && \
     apt-get -qq --yes install ${RUNTIME_PACKAGES} && \
     apt-get -qq --yes autoremove --purge && \
     apt-get -qq --yes clean all && \
-    rm -rf "/usr/share/doc/"* \
-           "/usr/src/"* \
-           "/var/cache/"* \
-           "/var/lib/apt/lists/"* \
-           "/var/log/"{apt/*,dpkg.log} \
-           "/tmp/"*
+    rm -rf /usr/share/doc/* \
+           /usr/src/* \
+           /var/cache/* \
+           /var/lib/apt/lists/* \
+           /var/log/{apt/*,dpkg.log} \
+           /tmp/*
 
-
-ENTRYPOINT ["/usr/bin/sslscan"]
-CMD ["--help"]
+ENTRYPOINT [ "/usr/bin/sslscan" ]
+CMD [ "--help" ]