feat: rebuild workspaces as the reference plugin

docker/Dockerfile

@@ -66,6 +66,9 @@ WORKDIR /app
 RUN apt-get update && apt-get install -y python3 make g++ && rm -rf /var/lib/apt/lists/*
 
 COPY package*.json ./
+# The plugin SDK is a workspace package core imports at runtime, so npm ci
+# has to see it to link it and install its dependencies.
+COPY packages/plugin-sdk/package.json ./packages/plugin-sdk/package.json
 COPY .npmrc ./
 COPY vendor ./vendor
 
@@ -106,6 +109,9 @@ COPY --chown=node:node --from=backend-builder /app/dist/backend ./dist/backend
 # Bundled first-party plugins (ssh-terminal). Resolved as dist/plugins by
 # getBundledPluginsDir(); without this the terminal never starts.
 COPY --chown=node:node --from=backend-builder /app/dist/plugins ./dist/plugins
+# Target of node_modules/@termix/plugin-sdk, which core and the plugins import.
+COPY --chown=node:node --from=backend-builder /app/packages/plugin-sdk/package.json ./packages/plugin-sdk/package.json
+COPY --chown=node:node --from=backend-builder /app/packages/plugin-sdk/dist ./packages/plugin-sdk/dist
 COPY --chown=node:node --from=opkssh-downloader /opkssh /app/opkssh-bundled
 COPY --chown=node:node package.json ./
 # Schema for Postgres and MySQL. Unused by the default SQLite deployment, which