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6.7: Additional Dockerfile Features

Additional Dockerfile Features

There are some additional features of Dockerfiles that are not shown in the example applications but are worth knowing about. These are highlighted in Dockerfile.sample and the corresponding build / run commands in the Makefile

# syntax=docker/dockerfile:1.5
# escape=\
# ^ OPTIONAL "directives" (must be at top if used)


# ARG is the only instruction that can come before FROM
# ARGs can be overriden at build time
# > docker build --build-arg BASE_VERSION=19.3 .


LABEL org.opencontainers.image.authors=""

RUN echo "Hey Team 👋 (shell form)"
RUN ["echo", "Hey Team 👋 (exec form)"]

# Heredocs allow for specifying multiple commands to
# be run within a single step, across multiple lines
# without lots of && and \
apt update
apt install iputils-ping -y

# --mount allows for mounting additional files
# into the build context
# RUN --mount=type=bind ...
# RUN --mount=type=cache ...
# RUN --mount=type=ssh ...
RUN --mount=type=secret,id=secret.txt,dst=/container-secret.txt \
  echo "Run the command that requires access to the secret here"

# Available only at build time
# (Still in image metadata though...)

# Available at build and run time

# Set the default working directory
# Use the convention of your language/framework
WORKDIR path/to/the/working/directory

ENTRYPOINT [ "echo", "Hey Team 👋 (entrypoint)" ]
CMD [ "+ (cmd)" ]
  1. Parser directives: Specify the particular Dockerfile syntax being used or modify the escape character.
  2. ARG: Enables setting variables at build time that do not persist in the final image (but can be seen in the image metadata).
  3. Heredocs syntax: Enables multi-line commands within a Dockerfile.
  4. Mounting secrets: Allows for providing sensitive credentials required at build time while keeping them out of the final image.
  5. ENTRYPOINT + CMD: The interaction between ENTRYPOINT and CMD can be confusing. Depending on whether arguments are provided at runtime one or more will be used.
    • See the examples by running make run-sample-entrypoint-cmd.
  6. buildx (multi-architecture images): You can use a feature called buildx to create images for multiple architectures from a single Dockerfile. This video goes into depth on that topic: