docker images: clean kill using tini

- PID 1 does not have default signal handlers which causes
unclean shutdown of processes
Signed-off-by: Lakshya Singh <lakshay.singh1108@gmail.com>

bitcoind/Dockerfile

@@ -103,7 +103,8 @@ RUN apk --no-cache add \
   boost-thread \
   sqlite-dev \
   libevent \
-  libzmq
+  libzmq \
+  tini
 
 ARG BITCOIN_VERSION
 ARG BITCOIN_PREFIX=/opt/bitcoin-${BITCOIN_VERSION}
@@ -129,7 +130,7 @@ VOLUME ["${BITCOIN_DATA}"]
 
 USER bitcoin
 
-ENTRYPOINT ["/entrypoint.sh"]
+ENTRYPOINT ["/sbin/tini", "--", "/entrypoint.sh"]
 
 RUN bitcoind -version | grep "Bitcoin Core version v${BITCOIN_VERSION}"