Install libicu in Docker images

[babolivier]

Add a dependency to libicu-dev and pkg-config to the Docker images, to help with Synapse's user search as per matrix-org/synapse#14464.

Required by matrix-org/synapse#14464

CI for Synapse is failing because (obviously) the matrixdotorg/sytest-synapse image does not have the libicu-dev dependency (and branch matching means it's trying to test against a code base that requires it).

[DMRobertson]

CI for Synapse is failing because (obviously) the matrixdotorg/sytest-synapse image does not have the libicu-dev dependency (and branch matching means it's trying to test against a code base that requires it).

Do we not build that image as part of CI? Surprising! (Maybe I screwed up porting from buildkite)

[babolivier]

Do we not build that image as part of CI? Surprising! (Maybe I screwed up porting from buildkite)

I don't remember the specifics of the config we used to have on BK, what I can see is that currently we build and push on pushes to develop (or when manually running the workflow): https://github.com/matrix-org/sytest/blob/develop/.github/workflows/docker.yml. It also doesn't seem like we push conditionally (i.e. we always push when we build).