dockerfile note

dockerfile note
reference

summary

defination
docker can build images automatically by reading the instructions from a dockerfile. dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image.
usage
the docker build command builds an image from a dockerfile and context. the build's context is the set of files at a specified location PATH or URL. warning: don't use PATH /, because it can transfer the entire contents of your hard drive to the docker daemon.
docker build -f /home/vickey/dockerfile .
format
a. INSTRUCTION is not case-sensitive but convention is UPPERCASE. e.g: FROM nginx:1.13
b. must start with FROM.
c. docker treat lines begin with # as a comment.
parser directive
reference
a. parser directive is not case-sentive but convention is lowercase and must at the first line of dockerfile e.g: # directive=value then the next line is FROM nginx:1.13
b. can't repeat
escape
in linux default is , " ` " in windows
variable replacement
${variable:-word} indicates that if variable is set then the result will be that value. If variable is not set then word will be the result.
${variable:+word} indicates that if variable is set then word will be the result, otherwise the result is the empty string.
.dockerignore file
Before the docker CLI sends the context to the docker daemon, it looks for a file named .dockerignore in the root directory of the context. If this file exists, the CLI modifies the context to exclude files and directories that match patterns in it
FROM
The tag or digest values are optional. If you omit either of them, the builder assumes a latest tag by default
FROM buildpack-deps:jessie

RUN

echo $HOME'```
equivalent to following
`RUN /bin/bash -c 'source $HOME/.bashrc; echo $HOME'`

CMD
There can only be one CMD instruction in a Dockerfile. If you list more than one CMD then only the last CMD will take effect
CMD echo "This is a test." | wc -
LABEL
A LABEL is a key-value pair.
LABEL version="1.0"
To view an image’s labels, use the docker inspect command
docker inspect docker container name
EXPOSE
By default, EXPOSE assumes TCP. You can also specify UDP:
EXPOSE 80/udp
or publish port when run images
docker run -p 80:80/tcp -p 80:80/udp image_name
ENV
The entire string after the first space will be treated as the - including whitespace characters. allows for multiple variables to be set at one time
```
ENV myDog Rex The Dog```
equivalent ti `ENV myName = John Doe`
```
ADD
The ADD instruction copies new files, directories or remote file URLs from and adds them to the filesystem of the image at the path
COPY
The COPY instruction copies new files or directories from <src> and adds them to the filesystem of the container at the path <dest>.
COPY VS ADD
VOLUME
The VOLUME instruction creates a mount point with the specified name and marks it as holding externally mounted volumes from native host or other containers. The value can be a JSON array, VOLUME ["/var/log/"], or a plain string with multiple arguments, such as VOLUME /var/log or VOLUME /var/log/var/db
WORKDIR
The WORKDIR instruction sets the working directory for any RUN, CMD, ENTRYPOINT, COPY and ADD instructions that follow it in the Dockerfile
```
RUN pwd```
```