dev: client distribution build script

[csmig]

• Changes the client source directory from clients/extjs to client/src
• Adds an empty client distribution directory client/dist
• Adds a build script client/build.sh that creates the minified web client distribution.
• Changes Dockerfile to copy client/dist (instead of clients/extjs) to ./client
• Adds a reusable GitHub workflow build-client.yml that calls client/build.sh.
• Changes the workflow pub-docker.yml to need and use build-client.yml prior to building container images
• Changes the workflow pub-docker.yml to specify the target DockerHub repository with workflow env DOCKERHUB_REPOSITORY
• Changes the workflow pub-docker.yml to checkout the repo with fetch-depth: 0 which removes the need to execute git fetch --prune --unshallow in a later step