complement.sh: don't use remote base synapse image

[AndrewFerr]

This PR is because I hit issues with the Complement script's attempt to build the synapse-workers image, as Docker refused to build it from my local matrixdotorg/synapse image & insisted on using the latest remote synapse image from Docker Hub instead.

As it turns out, this was because my Docker build driver was something other than docker, which is what's needed to let FROM <image> prefer local images: https://github.com/docker/buildx/issues/159#issuecomment-539238262

Reinstalling Docker from upstream repos (as opposed to Fedora's) lets the workers image build as intended, meaning this PR is only needed for compatibility with other build drivers.

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[AndrewFerr]

This PR is because I hit issues with the Complement script's attempt to build the synapse-workers image, as Docker refused to build it from my local matrixdotorg/synapse image & insisted on using the latest remote synapse image from Docker Hub instead.

As it turns out, this was because my Docker build driver was something other than docker, which is what's needed to let FROM <image> prefer local images: https://github.com/docker/buildx/issues/159#issuecomment-539238262

Reinstalling Docker from upstream repos (as opposed to Fedora's) lets the workers image build as intended, meaning this PR is only needed for compatibility with other build drivers.

[AndrewFerr]

Closing this because the root cause of my build issues was my Docker configuration: https://github.com/element-hq/synapse/pull/18210#discussion_r2499646952

This PR could still be used to make builds compatible with the kind of config I had, but it's not the responsibility of this repo to cover non-standard configs.