Building from sources miss packr2 steps

[arapaho]

Describe the bug
Building from source is broken: the resulting hydra binary is not usable to missing schema.

The procedure https://www.ory.sh/hydra/docs/install#building-from-source does not contain the step to install packr2 binary beforehand and the Makefile does not include the necessary calls to packr2 in install-stable and install targets.

Reproducing the bug

Steps to reproduce the behavior:

1. Clone repository: git checkout -b v1.4.10 https://github.com/ory/hydra
2. Build hydra: make install-stable
3. Rename .schema directory located in sources: mv .schema{,.test}
4. Start hydra with a test configuration file $(go env GOPATH)/bin/hydra --config /path/to/test_config.yml serve all and get the error.

Server logs

FATA[0000] Unable to open configuration JSON Schema.     error="stat /path/to/cloned/hydra/.schema/config.schema.json: no such file or directory"`

Server configuration

dsn: memory
log:
    level: debug
oauth2:
    expose_internal_errors: true
    hashers:
        bcrypt:
            cost: 10
secrets:
    cookie:
    - plopplopekujghibGajsiod8Ob
    system:
    - hahahaekujghibGajsiod8Ob
serve:
    admin:
        host: 127.0.0.1
        port: 4445
    public:
        cors:
            enabled: true
        host: 127.0.0.1
        port: 4444
strategies:
    scope: exact
ttl:
    access_token: 3m
    auth_code: 1m
    id_token: 3m
    login_consent_request: 1h
    refresh_token: 10m

Expected behavior

Resulting binary must run without the sources being present.

Environment

• Version: v1.4.10
• Environment: Debian testing and Ubuntu 18.04

Additional context

The problem has been discovered while building Ory Hydra with the goal of producing internal Debian/Ubuntu packages.