docker: add script help message for macOS

Problem: recently, several core developers have noted that the
docker-run-checks.sh script borks when command-line options that specify
values do so with an equals sign rather than a space on macOS.

Add a help message to show up on macOS reminding devs to strip all '='
and replace them with spaces in their command-line arguments if BSD
getopt is used. Also, give users the option to set an environment
variable to tell the script on macOS that it should assume GNU getopt
is being used ('FORCE_GNU_GETOPT=1').

src/test/docker/docker-run-checks.sh

@@ -27,7 +27,7 @@ die() { echo -e "$prog: $@"; exit 1; }
 #
 declare -r long_opts="help,quiet,interactive,image:,flux-security-version:,jobs:,no-cache,no-home,distcheck,tag:,build-directory:,install-only,no-poison,recheck,unit-test-only,quick-check,inception,platform:,workdir:,system"
 declare -r short_opts="hqIdi:S:j:t:D:Prup:"
-declare -r usage="
+declare usage="
 Usage: $prog [OPTIONS] -- [CONFIGURE_ARGS...]\n\
 Build docker image for CI builds, then run tests inside the new\n\
 container as the current user and group.\n\
@@ -59,9 +59,14 @@ Options:\n\
 "
 
 # check if running in OSX
-if [[ "$(uname)" == "Darwin" ]]; then
+if [[ "$(uname)" == "Darwin" ]] && [[ $FORCE_GNU_GETOPT != 1 ]]; then
     # BSD getopt
     GETOPTS=`getopt $short_opts -- $*`
+    EXTRA_MACOS_STUFF="\n\
+    You are using BSD getopt on macOS. BSD getopt does not recognize '='\n\
+    between options. Use a space instead. If gnu-getopt is first in your\n\
+    PATH, force the script to use that by setting FORCE_GNU_GETOPT=1.\n"
+    usage=${usage}${EXTRA_MACOS_STUFF}
 else
     # GNU getopt
     GETOPTS=`getopt -u -o $short_opts -l $long_opts -n $prog -- $@`