Use JSON notation for Dockerfile CMD arguments

Reference:
- https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#cmd

> The `CMD` instruction should be used to run the software contained in
your image, along with any arguments. `CMD` should almost always be used
in the form of `CMD ["executable", "param1", "param2"…]`

test/Dockerfile-alpine

@@ -18,4 +18,4 @@ RUN /bats-core/install.sh /usr/local
 
 # Use supervisord to run nginx in the background
 COPY ./test/test-config/alpine-supervisord.conf /etc/supervisord.conf
-CMD tail -f /dev/null
+CMD [ "tail", "-f", "/dev/null" ]

test/Dockerfile-centos6

@@ -23,4 +23,4 @@ RUN /bats-core/install.sh /usr/local
 EXPOSE 80 443
 
 # Run eternal loop - for testing
-CMD tail -f /dev/null
+CMD [ "tail", "-f", "/dev/null" ]

test/Dockerfile-centos7-staging

@@ -24,4 +24,4 @@ RUN /bats-core/install.sh /usr/local
 EXPOSE 80 443
 
 # Run eternal loop - for testing
-CMD tail -f /dev/null
+CMD [ "tail", "-f", "/dev/null" ]

test/Dockerfile-debian

@@ -16,4 +16,4 @@ RUN git clone --depth 1 https://github.com/bats-core/bats-assert /bats-assert
 RUN /bats-core/install.sh /usr/local
 
 # Run eternal loop - for testing
-CMD tail -f /dev/null
+CMD [ "tail", "-f", "/dev/null" ]

test/Dockerfile-ubuntu

@@ -22,4 +22,4 @@ RUN git clone --depth 1 https://github.com/bats-core/bats-assert /bats-assert
 RUN /bats-core/install.sh /usr/local
 
 # Run eternal loop - for testing
-CMD tail -f /dev/null
+CMD [ "tail", "-f", "/dev/null" ]

test/Dockerfile-ubuntu-staging

@@ -26,4 +26,4 @@ RUN git clone --depth 1 https://github.com/bats-core/bats-assert /bats-assert
 RUN /bats-core/install.sh /usr/local
 
 # Run eternal loop - for testing
-CMD tail -f /dev/null
+CMD [ "tail", "-f", "/dev/null" ]

test/Dockerfile-ubuntu16

@@ -18,4 +18,4 @@ RUN git clone --depth 1 https://github.com/bats-core/bats-assert /bats-assert
 RUN /bats-core/install.sh /usr/local
 
 # Run eternal loop - for testing
-CMD tail -f /dev/null
+CMD [ "tail", "-f", "/dev/null" ]

test/Dockerfile-ubuntu18

@@ -23,4 +23,4 @@ RUN /bats-core/install.sh /usr/local
 EXPOSE 80 443
 
 # Run eternal loop - for testing
-CMD tail -f /dev/null
+CMD [ "tail", "-f", "/dev/null" ]