Update "gpg" invocations to use "--batch"

See https://bugs.debian.org/913614

README.md

@@ -188,7 +188,7 @@ RUN set -x \
 	&& curl -fSL "https://github.com/krallin/tini/releases/download/$TINI_VERSION/tini" -o /usr/local/bin/tini \
 	&& curl -fSL "https://github.com/krallin/tini/releases/download/$TINI_VERSION/tini.asc" -o /usr/local/bin/tini.asc \
 	&& export GNUPGHOME="$(mktemp -d)" \
-	&& gpg --keyserver ha.pool.sks-keyservers.net --recv-keys 6380DC428747F6C393FEACA59A84159D7001A4E5 \
+	&& gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys 6380DC428747F6C393FEACA59A84159D7001A4E5 \
 	&& gpg --batch --verify /usr/local/bin/tini.asc /usr/local/bin/tini \
 	&& rm -r "$GNUPGHOME" /usr/local/bin/tini.asc \
 	&& chmod +x /usr/local/bin/tini \
@@ -256,7 +256,7 @@ The `Dockerfile` should be written to help mitigate man-in-the-middle attacks du
 	RUN curl -fSL "https://www.python.org/ftp/python/$PYTHON_VERSION/Python-$PYTHON_VERSION.tar.xz" -o python.tar.xz \
 	    && curl -fSL "https://www.python.org/ftp/python/$PYTHON_VERSION/Python-$PYTHON_VERSION.tar.xz.asc" -o python.tar.xz.asc \
 	    && export GNUPGHOME="$(mktemp -d)" \
-	    && gpg --keyserver ha.pool.sks-keyservers.net --recv-keys 97FC712E4C024BBEA48A61ED3A5CA953F73C700D \
+	    && gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys 97FC712E4C024BBEA48A61ED3A5CA953F73C700D \
 	    && gpg --batch --verify python.tar.xz.asc python.tar.xz \
 	    && rm -r "$GNUPGHOME" python.tar.xz.asc \
 	    # install

bashbrew/Dockerfile.release

@@ -30,7 +30,7 @@ ENV MANIFEST_TOOL_GPG_KEY 27F3EA268A97867EAF0BD05C0F386284C03A1162
 RUN set -euxo pipefail; \
 	\
 	export GNUPGHOME="$(mktemp -d)"; \
-	gpg --keyserver ha.pool.sks-keyservers.net --recv-keys "$MANIFEST_TOOL_GPG_KEY"; \
+	gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys "$MANIFEST_TOOL_GPG_KEY"; \
 	\
 	mkdir -p bin; \
 	\