let a stack compute its settings without make

A stack can now ship <name>.env and <name>.sh next to its compose files. The
hook is sourced with the tag helpers available, which is what the computing
.mk files of the catalogue were using make for: 29 of its 44 .mk files only
exist to build variables like the fabio tags.

Converting stack/host/fabio.mk by hand gives byte-identical output for the
route tag, and drops a trailing comma the make version left in the listener
list.

Also: APP_HOST and APP_URI are computed (the tag helpers build on them),
--color controls the escape codes rather than always emitting them, and
make test-portability runs the CLI under busybox ash and dash.

Makefile

@@ -1,4 +1,4 @@
-DEV_TARGETS := test test-unit test-golden test-integration lint golden-record
+DEV_TARGETS := test test-unit test-golden test-integration test-portability lint golden-record
 ifneq ($(filter $(DEV_TARGETS),$(MAKECMDGOALS)),)
 
 SHELLSPEC ?= shellspec
@@ -16,6 +16,17 @@ test-integration: ## Run tests needing a real docker daemon
 	MYOS_INTEGRATION=1 $(SHELLSPEC) spec/integration
 golden-record: ## Re-record golden expectations from the legacy engine
 	spec/golden/record.sh $(CASES)
+test-portability: ## Run the CLI under the /bin/sh of Alpine and Debian
+	@for img in alpine:3.20 debian:13-slim; do \
+	  printf '%s: ' "$$img"; \
+	  tar cf - --exclude .git . | docker run -i --rm "$$img" /bin/sh -c \
+	    'mkdir -p /myos && tar xf - -C /myos && cd /tmp && \
+	     export PATH=/myos/spec/support/bin:$$PATH && \
+	     /myos/bin/myos version >/dev/null && \
+	     /myos/bin/myos -C /myos/spec/fixtures/host-project -n up host >/dev/null && \
+	     echo ok'; \
+	done
+
 lint: ## shellcheck all shell sources
 	$(SHELLCHECK) -s bash myos spec/golden/record.sh spec/support/run.sh spec/support/bin/* $(wildcard bin/* lib/*.sh lib/cmd/*.sh install.sh)