Enable Travis autobuilds for 1.5

This is a simplified backport from master, whitout any testing.

tests/build.yml

@@ -0,0 +1,52 @@
+version: '3'
+
+services:
+
+  front:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}nginx:${MAILU_VERSION:-local}
+    build: ../core/nginx
+
+  imap:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}dovecot:${MAILU_VERSION:-local}
+    build: ../core/dovecot
+
+  smtp:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}postfix:${MAILU_VERSION:-local}
+    build: ../core/postfix
+
+  antispam:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}rspamd:${MAILU_VERSION:-local}
+    build: ../services/rspamd
+
+  antivirus:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}clamav:${MAILU_VERSION:-local}
+    build: ../optional/clamav
+
+  webdav:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}radicale:${MAILU_VERSION:-local}
+    build: ../optional/radicale
+
+  admin:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}admin:${MAILU_VERSION:-local}
+    build: ../core/admin
+
+  roundcube:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}roundcube:${MAILU_VERSION:-local}
+    build: ../webmails/roundcube
+
+  rainloop:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}rainloop:${MAILU_VERSION:-local}
+    build: ../webmails/rainloop
+
+  fetchmail:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}fetchmail:${MAILU_VERSION:-local}
+    build: ../services/fetchmail
+
+  none:
+    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}none:${MAILU_VERSION:-local}
+    build: ../core/none
+
+# Uncomment next section after docs Dockerfile creation.
+#  docs:
+#    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX}docs:${MAILU_VERSION:-local}
+#    build: ../docs