Add Dockerfile; add information into README file

Dockerfile

@@ -0,0 +1,20 @@
+FROM fedora:35
+
+RUN yum update -y
+
+RUN dnf -y groupinstall 'Development Tools'
+
+RUN dnf -y install bash-completion bzip2 gcc gcc-c++ git make ncurses-devel patch \
+    rsync tar unzip wget which diffutils python2 python3 perl-base \
+    perl-Data-Dumper perl-File-Compare perl-File-Copy perl-FindBin \
+    perl-Thread-Queue vim
+
+RUN useradd -m user && \
+    echo 'user ALL=NOPASSWD: ALL' > /etc/sudoers.d/user
+
+USER user
+WORKDIR /home/user
+
+# set dummy git config
+RUN git config --global user.name "user" && git \
+    config --global user.email "user@example.com"

README.md

@@ -1,2 +1,70 @@
-# openwrt-builder
-Build openWRT system image
+# Openwrt
+Build OpenWRT system image
+
+## How to
+
+Container image to build Openwrt image based on https://openwrt.org/docs/guide-developer/toolchain/install-buildsystem
+To build own container image:
+
+```shell
+sudo yum install -y git podman
+git clone https://github.com/danpawlik/openwrt-builder
+cd openwrt-builder
+podman build -t openwrt-builder -f Dockerfile
+```
+
+## Example
+
+how to use:
+```
+mkdir -p openwrt-builder
+podman run -it -u user -v $(pwd)/openwrt-builder/:/home/user:z,rw quay.io/dpawlik/openwrt:fedora35 /bin/bash
+```
+
+Then inside the container (from https://openwrt.org/docs/guide-developer/toolchain/use-buildsystem):
+
+```shell
+git clone https://git.openwrt.org/openwrt/openwrt.git
+cd openwrt
+git pull
+```
+
+Select a specific code revision:
+
+```shell
+git branch -a
+git tag
+git checkout v21.02.1
+```
+
+Update the feeds:
+
+```shell
+./scripts/feeds update -a
+./scripts/feeds install -a
+```
+
+Configure the firmware image and the kernel:
+
+```shell
+make menuconfig
+```
+
+If you have own config file, replace it. For example:
+
+```shell
+curl -SL https://downloads.openwrt.org/snapshots/targets/ath79/mikrotik/config.buildinfo > .config
+```
+
+Then:
+
+```shell
+# make kernel_menuconfig
+make -j4 kernel_menuconfig
+```
+
+Build the firmware image:
+
+```shell
+make -j $(nproc) defconfig download clean world
+```