[docker] fallback to alpine 3.13

https://wiki.alpinelinux.org/wiki/Draft_Release_Notes_for_Alpine_3.14.0#faccessat2

contrib/docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:latest
+FROM alpine:3.13
 LABEL authors "Mikal Villa <mikal@sigterm.no>, Darknet Villain <supervillain@riseup.net>"
 
 # Expose git branch, tag and URL variables as arguments
@@ -31,7 +31,6 @@ RUN apk update \
     && cd /tmp/build && git clone -b ${GIT_BRANCH} ${REPO_URL} \
     && cd i2pd \
     && if [ -n "${GIT_TAG}" ]; then git checkout tags/${GIT_TAG}; fi \
-    && g++ -dumpmachine \
     && make USE_UPNP=yes \
     && cp -R contrib/certificates /i2pd_certificates \
     && mkdir -p /usr/local/bin \