Doc: Add docker build instructions without git clone

Add instructions for building Docker image directly from GitHub. With modern enough Docker, this just works: ``` $ docker build --no-cache https://github.com/StevenBlack/hosts.git -t stevenblack-hosts [+] Building 70.7s (8/8) FINISHED => [internal] load git source https://github.com/StevenBlack/hosts.git 4.2s => [internal] load metadata for docker.io/library/python:3-alpine 5.7s => [1/5] FROM docker.io/library/python:3-alpine@sha256:dd4d2bd5b53d9b25a51da13addf2be586beebd5387e289e798e4083d94ca837a 5.5s => => resolve docker.io/library/python:3-alpine@sha256:dd4d2bd5b53d9b25a51da13addf2be586beebd5387e289e798e4083d94ca837a 0.1s => => sha256:9e03a80a1cc6bec92c07e5eb616b295ec3c1660faa90fd08a4a4ca92e03fafde 1.74kB / 1.74kB 0.0s => => sha256:1136aa64d9c50e7684573f089718ac9919575f9d39ea413410a0614552d006e5 4.71kB / 4.71kB 0.0s => => sha256:d17f077ada118cc762df373ff803592abf2dfa3ddafaa7381e364dd27a88fca7 4.20MB / 4.20MB 1.9s => => sha256:ce044e508c31b550012103ac403762a3f10a13a30aa1a69e117060b0d6d43a9d 457.91kB / 457.91kB 0.6s => => sha256:9def3ce2dae125b2efcb5100bde3ecee4d9b5f78b9088098753338af9e277661 13.48MB / 13.48MB 3.5s => => sha256:dd4d2bd5b53d9b25a51da13addf2be586beebd5387e289e798e4083d94ca837a 10.29kB / 10.29kB 0.0s => => sha256:9363636621cd74af602f2571bc4ee6ffe761c3ddcdbb85e4e29d3044e1d555e6 250B / 250B 1.0s => => extracting sha256:d17f077ada118cc762df373ff803592abf2dfa3ddafaa7381e364dd27a88fca7 0.2s => => extracting sha256:ce044e508c31b550012103ac403762a3f10a13a30aa1a69e117060b0d6d43a9d 0.1s => => extracting sha256:9def3ce2dae125b2efcb5100bde3ecee4d9b5f78b9088098753338af9e277661 0.8s => => extracting sha256:9363636621cd74af602f2571bc4ee6ffe761c3ddcdbb85e4e29d3044e1d555e6 0.0s => [2/5] RUN apk add --no-cache git sudo 28.3s => [3/5] COPY . /hosts 17.2s => [4/5] RUN pip install --no-cache-dir --upgrade -r /hosts/requirements.txt 7.9s => [5/5] WORKDIR /hosts 0.9s => exporting to image 0.8s => => exporting layers 0.7s => => writing image sha256:ee90f188f29247208d3eff1f455a2f1d7b27de292fa74850427b115b38105c9d 0.1s => => naming to docker.io/library/stevenblack-hosts 0.0s ```