Run arbitrary Borg commands with new "borgmatic borg" action (#425).

docs/Dockerfile

@@ -3,7 +3,7 @@ FROM python:3.8-alpine3.12 as borgmatic
 COPY . /app
 RUN pip install --no-cache /app && generate-borgmatic-config && chmod +r /etc/borgmatic/config.yaml
 RUN borgmatic --help > /command-line.txt \
-    && for action in init prune create check extract mount umount restore list info; do \
+    && for action in init prune create check extract export-tar mount umount restore list info borg; do \
            echo -e "\n--------------------------------------------------------------------------------\n" >> /command-line.txt \
            && borgmatic "$action" --help >> /command-line.txt; done
 

docs/how-to/develop-on-borgmatic.md

@@ -3,7 +3,7 @@ title: How to develop on borgmatic
 eleventyNavigation:
   key: Develop on borgmatic
   parent: How-to guides
-  order: 11
+  order: 12
 ---
 ## Source code
 

docs/how-to/run-arbitrary-borg-commands.md

@@ -0,0 +1,94 @@
+---
+title: How to run arbitrary Borg commands
+eleventyNavigation:
+  key: Run arbitrary Borg commands
+  parent: How-to guides
+  order: 10
+---
+## Running Borg with borgmatic
+
+Borg has several commands (and options) that borgmatic does not currently
+support. Sometimes though, as a borgmatic user, you may find yourself wanting
+to take advantage of these off-the-beaten-path Borg features. You could of
+course drop down to running Borg directly. But then you'd give up all the
+niceties of your borgmatic configuration. You could file a [borgmatic
+ticket](https://torsion.org/borgmatic/#issues) or even a [pull
+request](https://torsion.org/borgmatic/#contributing) to add the feature. But
+what if you need it *now*?
+
+That's where borgmatic's support for running "arbitrary" Borg commands comes
+in. Running Borg commands with borgmatic takes advantage of the following, all
+based on your borgmatic configuration files or command-line arguments:
+
+ * configured repositories (automatically runs your Borg command once for each
+   one)
+ * local and remote Borg binary paths
+ * SSH settings and Borg environment variables
+ * lock wait settings
+ * verbosity
+
+
+### borg action
+
+The way you run Borg with borgmatic is via the `borg` action. Here's a simple
+example:
+
+```bash
+borgmatic borg break-lock
+```
+
+(No `borg` action in borgmatic? Time to upgrade!)
+
+This runs Borg's `break-lock` command once on each configured borgmatic
+repository. Notice how the repository isn't present in the specified Borg
+options, as that part is provided by borgmatic.
+
+You can also specify Borg options for relevant commands:
+
+```bash
+borgmatic borg list --progress
+```
+
+This runs Borg's `list` command once on each configured borgmatic
+repository. However, the native `borgmatic list` action should be preferred
+for most use.
+
+What if you only want to run Borg on a single configured borgmatic repository
+when you've got several configured? Not a problem.
+
+```bash
+borgmatic borg --repository repo.borg break-lock
+```
+
+And what about a single archive?
+
+```bash
+borgmatic borg --archive your-archive-name list
+```
+
+### Limitations
+
+borgmatic's `borg` action is not without limitations:
+
+ * The Borg command you want to run (`create`, `list`, etc.) *must* come first
+   after the `borg` action. If you have any other Borg options to specify,
+   provide them after. For instance, `borgmatic borg list --progress` will work,
+   but `borgmatic borg --progress list` will not.
+ * borgmatic supplies the repository/archive name to Borg for you (based on
+   your borgmatic configuration or the `borgmatic borg --repository`/`--archive`
+   arguments), so do not specify the repository/archive otherwise.
+ * The `borg` action will not currently work for any Borg commands like `borg
+   serve` that do not accept a repository/archive name.
+ * Do not specify any global borgmatic arguments to the right of the `borg`
+   action. (They will be passed to Borg instead of borgmatic.) If you have
+   global borgmatic arguments, specify them *before* the `borg` action.
+ * Unlike other borgmatic actions, you cannot combine the `borg` action with
+   other borgmatic actions. This is to prevent ambiguity in commands like
+   `borgmatic borg list`, in which `list` is both a valid Borg command and a
+   borgmatic action. In this case, only the Borg command is run.
+ * Unlike normal borgmatic actions that support JSON, the `borg` action will
+   not disable certain borgmatic logs to avoid interfering with JSON output.
+
+In general, this `borgmatic borg` feature should be considered an escape
+valve—a feature of second resort. In the long run, it's preferable to wrap
+Borg commands with borgmatic actions that can support them fully.

docs/how-to/upgrade.md

@@ -3,7 +3,7 @@ title: How to upgrade borgmatic
 eleventyNavigation:
   key: Upgrade borgmatic
   parent: How-to guides
-  order: 10
+  order: 11
 ---
 ## Upgrading