Check for bash existence instead of only mentioning it in a comment

Author: codablock

core/src/test/resources/internal-port-check-dockerfile/Dockerfile-bash

@@ -1,4 +1,3 @@
-# If you ever upgrade this image, watch out for bash being installed by default in newer alpine base images
 FROM nginx:1.17-alpine
 
 RUN apk add bash

core/src/test/resources/internal-port-check-dockerfile/Dockerfile-nc

@@ -1,6 +1,8 @@
-# If you ever upgrade this image, watch out for bash being installed by default in newer alpine base images
 FROM nginx:1.17-alpine
 
+# If this fails, you ended up using a base image with bash installed. Consider removing /bin/bash in this case
+RUN if bash -c true &> /dev/null; then exit 1; fi
+
 # Make sure the /proc/net/tcp* check fails in this container
 RUN rm /usr/bin/awk
 

core/src/test/resources/internal-port-check-dockerfile/Dockerfile-tcp

@@ -1,6 +1,8 @@
-# If you ever upgrade this image, watch out for bash being installed by default in newer alpine base images
 FROM nginx:1.17-alpine
 
+# If this fails, you ended up using a base image with bash installed. Consider removing /bin/bash in this case
+RUN if bash -c true &> /dev/null; then exit 1; fi
+
 # Make sure the nc check fails in this container
 RUN rm /usr/bin/nc