docker: rootless image (#10154)

* docker: rootless image

* improve docs + remove check for write perm on custom

* add more info on ssh passtrough

* Add comment for internal ssh server in container config

Dockerfile.rootless

@@ -0,0 +1,68 @@
+
+###################################
+#Build stage
+FROM golang:1.15-alpine3.12 AS build-env
+
+ARG GOPROXY
+ENV GOPROXY ${GOPROXY:-direct}
+
+ARG GITEA_VERSION
+ARG TAGS="sqlite sqlite_unlock_notify"
+ENV TAGS "bindata timetzdata $TAGS"
+ARG CGO_EXTRA_CFLAGS 
+
+#Build deps
+RUN apk --no-cache add build-base git nodejs npm
+
+#Setup repo
+COPY . ${GOPATH}/src/code.gitea.io/gitea
+WORKDIR ${GOPATH}/src/code.gitea.io/gitea
+
+#Checkout version if set
+RUN if [ -n "${GITEA_VERSION}" ]; then git checkout "${GITEA_VERSION}"; fi \
+ && make clean-all build
+
+FROM alpine:3.12
+LABEL maintainer="maintainers@gitea.io"
+
+EXPOSE 2222 3000
+
+RUN apk --no-cache add \
+    bash \
+    ca-certificates \
+    gettext \
+    git \
+    gnupg
+
+RUN addgroup \
+    -S -g 1000 \
+    git && \
+  adduser \
+    -S -H -D \
+    -h /var/lib/gitea/git \
+    -s /bin/bash \
+    -u 1000 \
+    -G git \
+    git && \
+  echo "git:$(dd if=/dev/urandom bs=24 count=1 status=none | base64)" | chpasswd
+
+RUN mkdir -p /var/lib/gitea /etc/gitea
+RUN chown git:git /var/lib/gitea /etc/gitea
+
+COPY docker/rootless /
+COPY --from=build-env /go/src/code.gitea.io/gitea/gitea /usr/local/bin/gitea
+RUN chown root:root /usr/local/bin/* && chmod 755 /usr/local/bin/*
+
+USER git:git
+ENV GITEA_WORK_DIR /var/lib/gitea
+ENV GITEA_CUSTOM /var/lib/gitea/custom
+ENV GITEA_TEMP /tmp/gitea
+#TODO add to docs the ability to define the ini to load (usefull to test and revert a config)
+ENV GITEA_APP_INI /etc/gitea/app.ini
+ENV HOME "/var/lib/gitea/git"
+VOLUME ["/var/lib/gitea", "/etc/gitea"]
+WORKDIR /var/lib/gitea
+
+ENTRYPOINT ["/usr/local/bin/docker-entrypoint.sh"]
+CMD []
+