Run Travis tests in Docker without building image

Tests are now run on Travis using a plain PostgreSQL Docker image,
requiring no Docker build step or other dependencies on the host. The
source directory is mounted into the running image and the build and
test steps are run in the container via `docker exec`. This simplifies
the build process, obviates the need fo building a new test Docker
image, and uses Postgres tools (such as psql and pgdump) from the
image for consistency.

.travis.yml

@@ -8,18 +8,16 @@ services:
   - docker
 
 before_install:
-  - sudo add-apt-repository "deb http://apt.postgresql.org/pub/repos/apt/ trusty-pgdg main"
-  - wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
-  - sudo apt-get update
-  - sudo apt-get install postgresql-server-dev-9.6 postgresql-client-9.6
-  - sudo ln -sf /usr/lib/postgresql/9.6/bin/pg_dump /usr/bin/pg_dump
-  - sudo ln -sf /usr/lib/postgresql/9.6/bin/pg_restore /usr/bin/pg_restore
-  - pg_dump -V
-  - pg_restore -V
+  - docker run -d --name pgbuild -v ${TRAVIS_BUILD_DIR}:/build postgres:9.6.2-alpine
 
-install: make -f docker.mk build-image
+install:
+  - docker exec -it pgbuild /bin/sh -c "apk add --no-cache --virtual .build-deps coreutils dpkg-dev gcc libc-dev make util-linux-dev diffutils && make -C /build install && chown -R postgres:postgres /build/"
 
-script: make -f docker.mk test
+script:
+  - docker exec -u postgres -it pgbuild /bin/sh -c "make -C /build installcheck TEST_INSTANCE_OPTS='--temp-instance=/tmp/pgdata --temp-config=/build/test/postgresql.conf'"
 
-after_failure: 
-  - cat test/regression.diffs
+after_failure:
+  - docker exec -it pgbuild cat /build/test/regression.diffs
+
+after_script:
+  - docker rm -f pgbuild