Make generate-frozen-requirements reproducible through container

This allows to build the container that will call the generate-frozen-requirements
script.

That will be usable from ci or other sysadm.

Use:
```
cd swh-apps/scripts
docker build -t build-deps .
cd ../../swh-apps
docker run -v $PWD:/src --user 1000 build-deps $APP
```

scripts/Dockerfile

+# build-deps container to call scripts/generate-frozen-requirements.txt on an
+# application (e.g. swh-loader-svn, swh-loader-git, ...)
+FROM python:3.10-bullseye
+
+# First install the runtime deps of all swh applications
+RUN apt-get -y update && \
+    apt-get -y upgrade && \
+    apt-get install -y \
+      # objstorage deps (winery)
+      libcmph-dev \
+      # journal (kafka)
+      librdkafka-dev \
+      # svn loader (subvertpy)
+      libsvn-dev libapr1-dev libaprutil1-dev \
+      libpq-dev \
+      python3-venv \
+    && \
+    apt clean
+
+RUN mkdir -p /opt/scripts/
+COPY common.py /opt/scripts/
+COPY generate-frozen-requirements /opt/scripts/
+RUN chmod +x /opt/scripts/generate-frozen-requirements
+
+# This expects the /src/ to be mounted on swh-apps repository folder at runtime
+ENV SWH_APPS_DIR=/src/apps
+
+WORKDIR /opt/scripts
+ENTRYPOINT [ "./generate-frozen-requirements" ]

scripts/generate-frozen-requirements

@@ -5,16 +5,15 @@
 # License: GNU General Public License v3 or later
 # See top-level LICENSE file for more information
 
-"""Generate the requirements-frozen.txt file for a given app"""
+"""Generate the requirements-frozen.txt file for given app(s)"""
 
+import os
 import pathlib
 import subprocess
 import sys
 import tempfile
 from venv import EnvBuilder
 
-from common import APPS_DIR
-
 
 class AppEnvBuilder(EnvBuilder):
     """A virtualenv builder specialized for our usecase"""
@@ -49,11 +48,16 @@ def usage():
     print(f"Usage: {sys.argv[0]} app1 ... appN", file=sys.stderr)
 
 
-def generate_requirements_frozen(app):
+def generate_requirements_frozen(app: str, absolute_apps_dirpath: pathlib.Path) -> None:
     """Generate the ``requirements-frozen.txt`` file out of the
-    ``requirements.txt`` file present in the ``app`` directory"""
+    ``requirements.txt`` file present in the ``app`` directory
+
+    Args:
+        app: Name of the application to generate the frozen dependencies set
+        apps_dir: Absolute folder holding all application declarations
+    """
 
-    app_dir = APPS_DIR / app
+    app_dir = absolute_apps_dirpath / app
     src_req_file = app_dir / "requirements.txt"
     dst_req_file = app_dir / "requirements-frozen.txt"
 
@@ -80,5 +84,13 @@ if __name__ == "__main__":
         usage()
         sys.exit(2)
 
+    apps_dir = os.environ.get("SWH_APPS_DIR")
+    if not apps_dir:
+        from common import APPS_DIR
+
+        absolute_apps_dirpath = APPS_DIR
+    else:
+        absolute_apps_dirpath = pathlib.Path(apps_dir)
+
     for app in sys.argv[1:]:
-        generate_requirements_frozen(app)
+        generate_requirements_frozen(app, absolute_apps_dirpath)