diff --git a/containers/zammad/Dockerfile b/containers/zammad/Dockerfile
index b633d45..7a9c5b0 100644
--- a/containers/zammad/Dockerfile
+++ b/containers/zammad/Dockerfile
@@ -25,13 +25,13 @@ LABEL org.label-schema.build-date="$BUILD_DATE" \
 RUN BUILD_DEPENDENCIES="git build-essential libffi-dev libpq5 libpq-dev" \
     GOSU_DEPENDENCIES="ca-certificates wget" \
     set -ex \
-	  && apt-get update && apt-get install -y --force-yes --no-install-recommends $BUILD_DEPENDENCIES $GOSU_DEPENDENCIES && rm -rf /var/lib/apt/lists/* \
+	  && apt-get update && apt-get install -y --force-yes --no-install-recommends ${BUILD_DEPENDENCIES} ${GOSU_DEPENDENCIES} && rm -rf /var/lib/apt/lists/* \
 	  && wget -O /usr/local/bin/gosu "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$(dpkg --print-architecture)" \
 	  && wget -O /usr/local/bin/gosu.asc "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$(dpkg --print-architecture).asc" \
 	  && export GNUPGHOME="$(mktemp -d)" \
 	  && gpg --keyserver ha.pool.sks-keyservers.net --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4 \
 	  && gpg --batch --verify /usr/local/bin/gosu.asc /usr/local/bin/gosu \
-	  && rm -rf "$GNUPGHOME" /usr/local/bin/gosu.asc \
+	  && rm -rf "${GNUPGHOME}" /usr/local/bin/gosu.asc \
 	  && chmod +x /usr/local/bin/gosu \
 	  && gosu nobody true
 
@@ -49,4 +49,4 @@ RUN set -ex \
     && rm -r tmp/cache \
     && chown -R ${ZAMMAD_USER}:${ZAMMAD_USER} ${ZAMMAD_DIR}
 
-WORKDIR $ZAMMAD_DIR
+WORKDIR ${ZAMMAD_DIR}