build(jobnik-manager): build the image from a pruned workspace

[CptSchnitz]

Replace the single-repo Dockerfile with a shared, workspace-aware
definition at docker/backend.Dockerfile, following the reference
monorepo's backend image (minus its policy-engine download): prune
the workspace to the manager with turbo, install from a pnpm store
cache mount, build through turbo, then pnpm deploy only production
dependencies into the runtime image.

The runtime base image stays node:24-alpine, since the generated
Prisma client already targets its libc, and the migration
command-line tool is still pulled into the image via npx as the
final build step. That step now pins the major version
(npx prisma@6, matching the manager's own ^6.19.0 dependency) because
an unpinned npx now resolves Prisma 7, which dropped support for the
datasource url field this schema still uses -- caught by actually
building the image and running the migration against a throwaway
database, which is also this ticket's own verification.

pnpm deploy only carries a workspace package's dist output when the
package declares a files field (jobnik-manager never needed one
before, since it was git-cloned and built in place); without it, the
deploy silently dropped dist. Added files: ["dist/**/*"], matching
jobnik-openapi's existing convention.

While rebuilding dist standalone to verify the above, found that
assets:copy's migrations copyfiles invocation has been silently
flattening the migrations directory (copyfiles -f with an unquoted
glob loses recursion under the shell it runs in) and dropping every
migration file except migration_lock.toml. The old Dockerfile never
noticed because it copied migrations from source directly, bypassing
dist; now that dist is the deployed artifact, the copy had to be
fixed so 'prisma migrate deploy' has something to apply.

jobnik-manager also declares which Dockerfile builds it
(dockerfile field) so a future build matrix can find it without
anything hardcoded, and its own build:docker script now follows the
reference monorepo's convention of invoking docker buildx directly,
replacing a tsc-based script of the same name that no longer applied
under the new build and was never actually wired into the old
Dockerfile either.

Left a comment on ticket 06 flagging that its migration-runner
compose command needs the same prisma@6 pin.

Refs: .scratch/monorepo-migration/issues/05-container-image-pruned-workspace.md

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

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🟢	Statements	100% (🎯 80%)	784 / 784
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