Unable to build complement-synapse Docker image

[uhoreg]

I tried to build the complement-synapse image, as described in docker/README-testing.md, but when I ran docker build -t complement-synapse -f docker/complement/Dockerfile docker/complement, it failed, saying:

...
Step 4/21 : RUN echo "ALTER USER postgres PASSWORD 'somesecret'" | gosu postgres postgres --single
 ---> Running in cbf9ce044882
postgres: could not access the server configuration file "/var/lib/postgresql/data/postgresql.conf": No such file or directory
The command '/bin/sh -c echo "ALTER USER postgres PASSWORD 'somesecret'" | gosu postgres postgres --single' returned a non-zero code: 1

It appears that it did not persist the changes to /var/lib/postgresql/data that were made in the previous Dockerfile step did not get persisted, and so /var/lib/postgresql/data remains empty. This seems to be because /var/lib/postgresql/data is marked as a VOLUME in the postgres image, which means that changes to that directory are discarded.

If I combine the three RUN statements in the postgres section of the Dockerfile into a single line, then the build "succeeds", but of course the /var/lib/postgresql/data directory is still empty in the resulting image.

[uhoreg]

Using scripts-dev/complement.sh seems to build the image correctly, though.

[richvdh]

I think the difference is the use of BuildKit rather than legacy docker build. For some reason BuildKit includes /var/lib/postgresql/data in the image, so you need to set DOCKER_BUILDKIT=1 when building the image, and that should be mentioned in docker/README-testing.md. But you need to do that when building the other two images as well, so I'm surprised that you got this far without it.

A possibly-more-correct workaround would be to have the postgres_base image construct the database in a different directory.

[uhoreg]

I originally used the matrixdotorg/synapse image from Docker hub, so that's why I was able to get that image without DOCKER_BUILDKIT=1.