Docker: use a specific directory to build unit.

pkg/docker/template.Dockerfile

@@ -13,6 +13,8 @@ RUN set -ex \
     && apt-get update \
     && apt-get install --no-install-recommends --no-install-suggests -y ca-certificates mercurial build-essential libssl-dev libpcre2-dev curl pkg-config \
     && mkdir -p /usr/lib/unit/modules /usr/lib/unit/debug-modules \
+    && mkdir -p /usr/src/unit \
+    && cd /usr/src/unit \
     && hg clone -u @@VERSION@@-@@PATCHLEVEL@@ https://hg.nginx.org/unit \
     && cd unit \
     && NCPU="$(getconf _NPROCESSORS_ONLN)" \
@@ -52,7 +54,7 @@ RUN set -ex \
     && ./configure @@CONFIGURE@@ \
     && make -j $NCPU @@INSTALL@@ \
     && cd \
-    && rm -rf unit \
+    && rm -rf /usr/src/unit \
     && for f in /usr/sbin/unitd /usr/lib/unit/modules/*.unit.so; do \
         ldd $f | awk '/=>/{print $(NF-1)}' | while read n; do dpkg-query -S $n; done | sed 's/^\([^:]\+\):.*$/\1/' | sort | uniq >> /requirements.apt; \
        done \