Base gitea on busybox glibc image

Build gitea from busybox glibc image.
It gets a little smaller (230M instead of 270M).
Added bash, gitea warns if not found.

Dockerfile.base.busybox

@@ -0,0 +1,12 @@
+# syntax=docker/dockerfile:1
+# gentoo-base-busybox
+FROM docker.io/gentoo/stage3:nomultilib AS builder
+
+WORKDIR /workdir
+COPY --exclude=repos . .
+COPY repos /var/db/repos
+RUN echo 'FEATURES="-ipc-sandbox -network-sandbox -pid-sandbox"' >> /etc/portage/make.conf
+RUN --mount=type=cache,target=/workdir/cache,sharing=locked \
+	    /bin/bash src/bootstrap.sh -u -r /var/db/repos/ceamac -p ceamac:linux/arch/amd64/busybox -m make-docker.conf /mnt/gentoo -j12
+
+CMD ["/bin/sh"]