Set `-x` option in SHELL command in Dockerfile

Intead of running `set -x` in the RUN commands

Dockerfile

@@ -1,19 +1,18 @@
 FROM jedisct1/alpine-runit:latest
 MAINTAINER Frank Denis
+SHELL ["/bin/sh", "-x", "-c"]
 ENV SERIAL 3
 
 ENV BUILD_DEPS   make gcc musl-dev git libevent-dev expat-dev shadow autoconf file openssl-dev byacc linux-headers
 ENV RUNTIME_DEPS bash util-linux coreutils findutils grep openssl ldns ldns-tools libevent expat libexecinfo coreutils drill ca-certificates
 
-RUN set -x && \
-    apk --no-cache upgrade && apk add --no-cache $RUNTIME_DEPS && \
+RUN apk --no-cache upgrade && apk add --no-cache $RUNTIME_DEPS && \
     update-ca-certificates 2> /dev/null || true
 
 ENV UNBOUND_GIT_URL https://github.com/jedisct1/unbound.git
 ENV UNBOUND_GIT_REVISION 4edb15ba417c78710069a5be8be3a6b5d8bdba9c
 
-RUN set -x && \
-    apk add --no-cache $BUILD_DEPS && \
+RUN apk add --no-cache $BUILD_DEPS && \
     mkdir -p /tmp/src && \
     cd /tmp/src && \
     git clone --depth=1000 "$UNBOUND_GIT_URL" && \
@@ -31,8 +30,7 @@ RUN set -x && \
 
 ENV LIBSODIUM_GIT_URL https://github.com/jedisct1/libsodium.git
 
-RUN set -x && \
-    apk add --no-cache $BUILD_DEPS && \
+RUN apk add --no-cache $BUILD_DEPS && \
     mkdir -p /tmp/src && \
     cd /tmp/src && \
     git clone --depth=1 --branch stable "$LIBSODIUM_GIT_URL" && \
@@ -48,8 +46,7 @@ ENV DNSCRYPT_WRAPPER_GIT_BRANCH xchacha-stamps
 
 COPY queue.h /tmp
 
-RUN set -x && \
-    apk add --no-cache $BUILD_DEPS && \
+RUN apk add --no-cache $BUILD_DEPS && \
     mkdir -p /tmp/src && \
     cd /tmp/src && \
     git clone --depth=1 --branch="${DNSCRYPT_WRAPPER_GIT_BRANCH}" "${DNSCRYPT_WRAPPER_GIT_URL}" && \
@@ -67,8 +64,7 @@ RUN set -x && \
     apk del --purge $BUILD_DEPS && \
     rm -fr /tmp/* /var/tmp/*
 
-RUN set -x && \
-    echo rm -rf /tmp/* /var/tmp/* /usr/local/include
+RUN echo rm -rf /tmp/* /var/tmp/* /usr/local/include
 
 RUN mkdir -p \
     /etc/service/unbound \