dockerfile: add registry to base image specifier & force `rm` for `podman build`

```
STEP 1: FROM alpine:3.13.1 AS build
Error: error creating build container: (image name "alpine:3.13.1" is a short name and no search registries are defined in /etc/containers/registries.conf): [...]
```

CHANGELOG.md

@@ -8,6 +8,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 - dockerfile: split `pipenv install` into two stages to speed up image builds
 - dockerfile: `chmod` files copied from host to no longer require `o=rX` perms on host
+- dockerfile: add registry to base image specifier for `podman build`
+- dockerfile: add `--force` flag to `rm` invocation to avoid interactive questions while running `podman build`
 
 ## [0.5.0] - 2020-11-06
 ### Added

Dockerfile

@@ -2,7 +2,7 @@
 
 # not using python:3.*-alpine cause glib-dev package depends on python3
 # https://pkgs.alpinelinux.org/package/v3.11/main/aarch64/glib-dev
-ARG BASE_IMAGE=alpine:3.13.1
+ARG BASE_IMAGE=docker.io/alpine:3.13.1
 ARG SOURCE_DIR_PATH=/systemctl-mqtt
 
 
@@ -42,7 +42,7 @@ COPY --chown=build:nobody . $SOURCE_DIR_PATH
 RUN pipenv install --deploy --verbose \
     && pipenv graph \
     && pipenv run pip freeze \
-    && rm -r .git/ $PIPENV_CACHE_DIR \
+    && rm -rf .git/ $PIPENV_CACHE_DIR \
     && chmod -cR a+rX .
 
 # workaround for broken multi-stage copy