add the config and compose modules

config: dotenv files are parsed, never sourced, so a value can hold a # or a
$(...) without breaking or executing. The make engine included .env with
make syntax, which neither allowed.
compose: one call per project instead of one per sub-stack.

spec/unit/config_load_helper.sh

@@ -0,0 +1,5 @@
+#shellcheck shell=sh
+# Helper: myos_dotenv_load sets variables in the caller's scope.
+[ -n "${2:-}" ] && LOADED=$2
+myos_dotenv_load "$1"
+printf '%s\n' "$LOADED"

spec/unit/config_spec.sh

@@ -0,0 +1,82 @@
+#shellcheck shell=sh
+Include lib/str.sh
+Include lib/core.sh
+Include lib/tags.sh
+Include lib/config.sh
+
+Describe 'lib/config.sh'
+  setup() { MYOS_TMP=$(mktemp -d "${TMPDIR:-/tmp}/myos-cfg.XXXXXX"); }
+  cleanup() { rm -rf "$MYOS_TMP"; }
+  BeforeEach setup
+  AfterEach cleanup
+
+  Describe 'myos_dotenv_parse'
+    It 'keeps plain assignments, drops comments and blanks, trims around the ='
+      printf '# a comment\n\nFOO=bar\n  BAZ = qux \nnot an assignment\n' > "$MYOS_TMP/.env"
+      When call myos_dotenv_parse "$MYOS_TMP/.env"
+      The line 1 should equal "FOO=bar"
+      The line 2 should equal "BAZ=qux "
+      The lines of output should equal 2
+    End
+    It 'strips one layer of quotes'
+      printf 'A="quoted"\nB='"'"'single'"'"'\nC=bare\n' > "$MYOS_TMP/.env"
+      When call myos_dotenv_parse "$MYOS_TMP/.env"
+      The line 1 should equal "A=quoted"
+      The line 2 should equal "B=single"
+      The line 3 should equal "C=bare"
+    End
+    It 'keeps a value containing a hash, which the make include could not'
+      printf 'PASS=aa#bb\n' > "$MYOS_TMP/.env"
+      When call myos_dotenv_parse "$MYOS_TMP/.env"
+      The output should equal "PASS=aa#bb"
+    End
+    It 'never executes the file'
+      printf 'X=$(touch %s/pwned)\n' "$MYOS_TMP" > "$MYOS_TMP/.env"
+      When call myos_dotenv_parse "$MYOS_TMP/.env"
+      The output should equal 'X=$(touch '"$MYOS_TMP"'/pwned)'
+      The path "$MYOS_TMP/pwned" should not be exist
+    End
+    It 'is empty for a missing file'
+      When call myos_dotenv_parse "$MYOS_TMP/nope"
+      The output should equal ""
+    End
+  End
+
+  Describe 'myos_dotenv_load'
+    It 'sets the variables of the file'
+      printf 'LOADED=yes\n' > "$MYOS_TMP/.env"
+      When run source spec/unit/config_load_helper.sh "$MYOS_TMP/.env"
+      The output should equal "yes"
+    End
+    It 'never overrides a variable that already has a value'
+      printf 'LOADED=fromfile\n' > "$MYOS_TMP/.env"
+      When run source spec/unit/config_load_helper.sh "$MYOS_TMP/.env" preset
+      The output should equal "preset"
+    End
+  End
+
+  Describe 'myos_env_vars'
+    It 'collects the variables referenced by a compose file'
+      printf 'services:\n  a:\n    image: ${IMAGE}\n    environment:\n      X: ${FOO:-d}\n' > "$MYOS_TMP/c.yml"
+      When call myos_env_vars "$MYOS_TMP/c.yml"
+      The output should equal "FOO IMAGE"
+    End
+    It 'ignores the $$ compose escape'
+      printf 'services:\n  a:\n    command: echo $$HOME ${REAL}\n' > "$MYOS_TMP/c.yml"
+      When call myos_env_vars "$MYOS_TMP/c.yml"
+      The output should equal "REAL"
+    End
+    It 'is empty without files'
+      When call myos_env_vars
+      The output should equal ""
+    End
+  End
+
+  Describe 'myos_env_export'
+    It 'prints only the variables that have a value'
+      SET_ONE=x; SET_TWO=
+      When call myos_env_export SET_ONE SET_TWO SET_MISSING
+      The output should equal "SET_ONE=x"
+    End
+  End
+End