[TASK] Add verbose error when docker entrypoint fails (#799)

A common problem for macOS users running docker as non-root is that
omitting a '--user' option makes our entrypoint.sh script try to
on-the-fly create a matching user.

(See TYPO3BestPractices/tea#1490)

This patch adds a verbose error message to hint at the problem.

entrypoint.sh

@@ -105,8 +105,23 @@ if [ "${MY_UID}" -eq "0" ]; then
         fi
 
         addgroup typo3 --gid=$GID;
+
+        if [ $? -ne 0 ]; then
+            echo "Error: Failed to add group 'typo3' inside docker container."
+            echo "This can happen if docker does not run as root. Please add the"
+            echo "argument '--user=\$(id -u):\$(id -g)' to your 'docker run...' call."
+            exit 1
+        fi
+
         adduser -h $(pwd) -D -G typo3 --uid=$UID typo3;
 
+        if [ $? -ne 0 ]; then
+            echo "Error: Failed to add user 'typo3' inside docker container."
+            echo "This can happen if docker does not run as root. Please add the"
+            echo "argument '--user=\$(id -u):\$(id -g)' to your 'docker run...' call."
+            exit 1
+        fi
+
         # su behaves inconsistently with -c followed by flags
         # Workaround: run the entrypoint and commands as a standalone script
         echo "#!/usr/bin/env sh" > /usr/local/bin/invocation.sh