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.

skills/myos/references/authoring.md

@@ -6,6 +6,8 @@
 stack/<name>/<name>.yml          the services
 stack/<name>/<name>.local.yml    what only makes sense on a workstation (published ports…)
 stack/<name>/<name>.labels.yml   the registrator labels, so routing stays optional
+stack/<name>/<name>.env          plain settings: versions, defaults
+stack/<name>/<name>.sh           computed settings (fabio tags), no make needed
 stack/<name>/.env.dist           the variables it expects, with defaults
 stack/<name>/README.md           what it is and what it needs
 ```

skills/myos/references/conventions.md

@@ -99,6 +99,33 @@ parsed, never sourced, so a value may contain a `#` or a `$(...)` without
 breaking anything or being executed. The make engine included `.env` as a
 makefile, where both broke.
 
+## Per-stack settings
+
+A stack keeps its own settings next to its compose files:
+
+| file | for |
+|---|---|
+| `<name>.env` | plain values: versions, defaults |
+| `<name>.env.<env>` | the same, for one environment |
+| `<name>.sh` | values that have to be computed |
+| `<name>.mk` | the legacy make snippet; still read for its groups |
+
+A `.sh` hook is sourced with the myos helpers available, and sets variables
+directly. This is what lets a stack work on a machine that has no make:
+
+```sh
+# stack/host/fabio.sh
+HOST_FABIO_VERSION=${HOST_FABIO_VERSION:-1.6.3}
+HOST_FABIO_SERVICE_9998_NAME=${HOST_FABIO_SERVICE_9998_NAME:-fabio}
+HOST_FABIO_SERVICE_9998_AUTH=${HOST_FABIO_SERVICE_9998_AUTH:-default}
+HOST_FABIO_SERVICE_9998_TAGS=${HOST_FABIO_SERVICE_9998_TAGS:-$(myos_tagprefix HOST_FABIO 9998)}
+```
+
+Always write `${VAR:-default}` so the environment and the `.env` still win.
+Helpers available in a hook: `myos_tagprefix`, `myos_urlprefix`, `myos_uri`,
+`myos_url`, `myos_envprefix`, `myos_servicenvs`, `myos_var`, `myos_lower`,
+`myos_upper`.
+
 ## Groups
 
 A group is a lowercase name whose value lists stacks. It can live in a `.env`,