Fix docker commands

[mbochnak-sumo]

• fix docker build command in README.md
• fix dockerfile

Before:

1. incorrect command in README.md:

$ docker build --load -t mkdocs .
unknown flag: --load
See 'docker build --help'.

2. Wrong formatting in Dockerfile:

$ docker build -t mkdocs .
[+] Building 2.0s (7/9)
 => [internal] load build definition from Dockerfile                                                                                                                                                                                                   0.0s
 => => transferring dockerfile: 667B                                                                                                                                                                                                                   0.0s
 => [internal] load .dockerignore                                                                                                                                                                                                                      0.0s
 => => transferring context: 2B                                                                                                                                                                                                                        0.0s
 => [internal] load metadata for docker.io/circleci/python:3.9                                                                                                                                                                                         1.8s
 => [1/6] FROM docker.io/circleci/python:3.9@sha256:1cdee732a8e950196ef3f46b3122149ee3c2ef75b3d8e1ac34be90573c6660a8                                                                                                                                   0.0s
 => CACHED [2/6] RUN pip install mkdocs pymdown-extensions pygments                                                                                                                                                                                    0.0s
 => CACHED [3/6] WORKDIR /tmp                                                                                                                                                                                                                          0.0s
 => ERROR [4/6] RUN git clone https://github.com/pagerduty/mkdocs-theme-pagerduty  cd mkdocs-theme-pagerduty && python3 setup.py install
------
 > [4/6] RUN git clone https://github.com/pagerduty/mkdocs-theme-pagerduty  cd mkdocs-theme-pagerduty && python3 setup.py install:
#7 0.178 fatal: Too many arguments.

3. wrong permissions in Dockerfile:

$ docker build -t mkdocs .
[+] Building 1.5s (9/11)
 => [internal] load build definition from Dockerfile                                                                                                                                                                                                   0.0s
 => => transferring dockerfile: 675B                                                                                                                                                                                                                   0.0s
 => [internal] load .dockerignore                                                                                                                                                                                                                      0.0s
 => => transferring context: 2B                                                                                                                                                                                                                        0.0s
 => [internal] load metadata for docker.io/circleci/python:3.9                                                                                                                                                                                         0.8s
 => [1/8] FROM docker.io/circleci/python:3.9@sha256:1cdee732a8e950196ef3f46b3122149ee3c2ef75b3d8e1ac34be90573c6660a8                                                                                                                                   0.0s
 => CACHED [2/8] RUN pip install mkdocs pymdown-extensions pygments                                                                                                                                                                                    0.0s
 => CACHED [3/8] WORKDIR /tmp                                                                                                                                                                                                                          0.0s
 => CACHED [4/8] RUN git clone https://github.com/pagerduty/mkdocs-theme-pagerduty                                                                                                                                                                     0.0s
 => CACHED [5/8] WORKDIR /tmp/mkdocs-theme-pagerduty                                                                                                                                                                                                   0.0s
 => ERROR [6/8] RUN python3 setup.py install
------
 > [6/8] RUN python3 setup.py install:
#9 0.450 running install
#9 0.522 error: can't create or remove files in install directory

4. wrong permissions in Dockerfile:

$ docker build -t mkdocs .
[+] Building 1.9s (11/11) FINISHED
 => [internal] load build definition from Dockerfile                                                                                                                                                                                                   0.0s
 => => transferring dockerfile: 682B                                                                                                                                                                                                                   0.0s
 => [internal] load .dockerignore                                                                                                                                                                                                                      0.0s
 => => transferring context: 2B                                                                                                                                                                                                                        0.0s
 => [internal] load metadata for docker.io/circleci/python:3.9                                                                                                                                                                                         0.8s
 => [1/8] FROM docker.io/circleci/python:3.9@sha256:1cdee732a8e950196ef3f46b3122149ee3c2ef75b3d8e1ac34be90573c6660a8                                                                                                                                   0.0s
 => CACHED [2/8] RUN pip install mkdocs pymdown-extensions pygments                                                                                                                                                                                    0.0s
 => CACHED [3/8] WORKDIR /tmp                                                                                                                                                                                                                          0.0s
 => CACHED [4/8] RUN git clone https://github.com/pagerduty/mkdocs-theme-pagerduty                                                                                                                                                                     0.0s
 => CACHED [5/8] WORKDIR /tmp/mkdocs-theme-pagerduty                                                                                                                                                                                                   0.0s
 => [6/8] RUN python3 setup.py install --user                                                                                                                                                                                                          0.8s
 => [7/8] WORKDIR /docs                                                                                                                                                                                                                                0.0s
 => ERROR [8/8] RUN useradd -m --uid 1000 mkdocs                                                                                                                                                                                                       0.2s
------
 > [8/8] RUN useradd -m --uid 1000 mkdocs:
#10 0.192 useradd: Permission denied.
#10 0.192 useradd: cannot lock /etc/passwd; try again later.

After

$ docker build -t mkdocs .
[+] Building 0.9s (12/12) FINISHED
 => [internal] load build definition from Dockerfile                                                                                                                                                                                                   0.0s
 => => transferring dockerfile: 722B                                                                                                                                                                                                                   0.0s
 => [internal] load .dockerignore                                                                                                                                                                                                                      0.0s
 => => transferring context: 2B                                                                                                                                                                                                                        0.0s
 => [internal] load metadata for docker.io/circleci/python:3.9                                                                                                                                                                                         0.8s
 => [1/8] FROM docker.io/circleci/python:3.9@sha256:1cdee732a8e950196ef3f46b3122149ee3c2ef75b3d8e1ac34be90573c6660a8                                                                                                                                   0.0s
 => CACHED [2/8] RUN pip install mkdocs pymdown-extensions pygments                                                                                                                                                                                    0.0s
 => CACHED [3/8] WORKDIR /tmp                                                                                                                                                                                                                          0.0s
 => CACHED [4/8] RUN git clone https://github.com/pagerduty/mkdocs-theme-pagerduty                                                                                                                                                                     0.0s
 => CACHED [5/8] WORKDIR /tmp/mkdocs-theme-pagerduty                                                                                                                                                                                                   0.0s
 => CACHED [6/8] RUN python3 setup.py install                                                                                                                                                                                                          0.0s
 => CACHED [7/8] WORKDIR /docs                                                                                                                                                                                                                         0.0s
 => CACHED [8/8] RUN useradd -m --uid 1000 mkdocs                                                                                                                                                                                                      0.0s
 => exporting to image                                                                                                                                                                                                                                 0.0s
 => => exporting layers                                                                                                                                                                                                                                0.0s
 => => writing image sha256:615b67cf9ea557319df1a872fdbde2990dc20b35082e1cb5c04c75a5b7fc3401                                                                                                                                                           0.0s
 => => naming to docker.io/library/mkdocs