add ability to read values from environment variables

It makes sense to read some values from environment. In particular this
feature is useful in case of running application in containers in ci/cd

See #108 for more details

src/ahriman/core/configuration/configuration.py

@@ -25,6 +25,7 @@ from collections.abc import Callable
 from pathlib import Path
 from typing import Any, Self
 
+from ahriman.core.configuration.shell_interpolator import ShellInterpolator
 from ahriman.core.exceptions import InitializeError
 from ahriman.models.repository_paths import RepositoryPaths
 
@@ -73,10 +74,16 @@ class Configuration(configparser.RawConfigParser):
             allow_no_value(bool, optional): copies ``configparser.RawConfigParser`` behaviour. In case if it is set
                 to ``True``, the keys without values will be allowed (Default value = False)
         """
-        configparser.RawConfigParser.__init__(self, allow_no_value=allow_no_value, converters={
-            "list": shlex.split,
-            "path": self._convert_path,
-        })
+        configparser.RawConfigParser.__init__(
+            self,
+            allow_no_value=allow_no_value,
+            interpolation=ShellInterpolator(),
+            converters={
+                "list": shlex.split,
+                "path": self._convert_path,
+            }
+        )
+
         self.architecture: str | None = None
         self.path: Path | None = None
         self.includes: list[Path] = []

src/ahriman/core/configuration/shell_interpolator.py

@@ -0,0 +1,51 @@
+#
+# Copyright (c) 2021-2023 ahriman team.
+#
+# This file is part of ahriman
+# (see https://github.com/arcan1s/ahriman).
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+import configparser
+import os
+
+from collections.abc import Mapping, MutableMapping
+from string import Template
+
+
+class ShellInterpolator(configparser.Interpolation):
+    """
+    custom string interpolator, because we cannot use defaults argument due to config validation
+    """
+
+    def before_get(self, parser: MutableMapping[str, Mapping[str, str]], section: str, option: str, value: str,
+                   defaults: Mapping[str, str]) -> str:
+        """
+        interpolate option value
+
+        Args:
+            parser(MutableMapping[str, Mapping[str, str]]): option parser
+            section(str): section name
+            option(str): option name
+            value(str): source (not-converted) value
+            defaults(Mapping[str, str]): default values
+
+        Returns:
+            str: substituted value
+        """
+        # At the moment it seems that it is the most legit way to handle environment variables
+        # Template behaviour is literally the same as shell
+        # In addition, we are using shell-like variables in some cases (see ``alpm.mirror`` option), thus we would like
+        # to keep them alive
+        return Template(value).safe_substitute(os.environ)