Convert CMD instructions to exec form for proper signal handling

- Updated 6 Dockerfiles to use JSON array syntax for CMD
- Ensures tail process runs as PID 1 and receives signals properly
- Improves container shutdown behavior and signal forwarding

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/Dockerfile-bash4-0

@@ -31,4 +31,4 @@ 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-bash4-2

@@ -31,4 +31,4 @@ 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-bash5-0

@@ -31,4 +31,4 @@ 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-centos7-dynu

@@ -30,4 +30,4 @@ 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-ubuntu-acmedns

@@ -36,4 +36,4 @@ RUN git clone 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-dynu

@@ -33,4 +33,4 @@ RUN git clone 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" ]