pmb.helpers.apk: support PMB_APK_NO_CACHE

Add a new environment variable that disables apk's caching feature for
space constrained environments such as the bpo image build jobs.

Currently we have a workaround in place in bpo: the apk cache is moved
to a tmpfs. But this is fragile and just disabling the apk cache is a
more elegant solution.

I've decided to make this an env var instead of a full pmbootstrap
option since this option is not relevant for normal users, only for CI
jobs in space constrained environments. Also we already have another
`PMB_APK_` env var.

Related: bpo issue 136
Part-of: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/merge_requests/2553

docs/environment_variables.md

@@ -14,6 +14,11 @@ APKINDEX results in a 404 not found error. This is used by
 the first time. For example if the `x86_64` repository was already built and
 published, but the `aarch64` repository wasn't published yet.
 
+## `PMB_APK_NO_CACHE`
+
+When this is set to `1`, pmbootstrap will disable apk's caching feature. This
+is used by bpo for image build jobs, so these jobs don't need as much space.
+
 ## `PMB_FDE_PASSWORD`
 
 This variable can be used to set the password when running `install --fde`. The

pmb/helpers/apk.py

@@ -173,10 +173,14 @@ def _prepare_cmd(command: Sequence[PathString], chroot: Chroot | None) -> list[s
                 str(chroot.path),
                 "--arch",
                 str(chroot.arch),
-                "--cache-dir",
-                str(cache_dir),
             ]
         )
+
+        if os.getenv("PMB_APK_NO_CACHE") == "1":
+            _command.extend(["--no-cache"])
+        else:
+            _command.extend(["--cache-dir", str(cache_dir)])
+
         local_repos = pmb.helpers.repo.urls(
             user_repository=config.work / "packages", mirrors_exclude=True
         )
@@ -212,7 +216,11 @@ def run(command: Sequence[PathString], chroot: Chroot, with_progress: bool = Tru
             raise RuntimeError(
                 "Encountered an 'apk add' command without --no-interactive! This is a bug."
             )
-        if "--cache-dir" not in _command:
+        if os.getenv("PMB_APK_NO_CACHE") == "1" and "--no-cache" not in _command:
+            raise RuntimeError(
+                "Encountered an 'apk add' command without --no-cache! This is a bug."
+            )
+        if os.getenv("PMB_APK_NO_CACHE") != "1" and "--cache-dir" not in _command:
             raise RuntimeError(
                 "Encountered an 'apk add' command without --cache-dir! This is a bug."
             )