Feature Request: Dockerfile ARG replacement should leave default value alone

[Vlatombe]

Is your feature request related to a problem?

This relates to #176

Following the documentation leads to replacing all arg entries with the given key, even those using the default value.

Using the example from the documentation

input

### File: Dockerfile.webapp
ARG GO_VERSION=1.15.8
FROM golang:"${GO_VERSION}-alpine" AS build
# Retrieve source code
WORKDIR /app
COPY . /app
# Build application
RUN go build -X "GoVersion=${GO_VERSION}" -o ./webapp

FROM ubuntu AS test
ARG GO_VERSION
ENV GO_VERSION=${GO_VERSION}
RUN ./run_tests.sh

FROM ubuntu:18.04 AS run
COPY --from=builder /app/webapp /usr/local/bin/webapp

becomes

### File: Dockerfile.webapp
ARG GO_VERSION=1.15.11 (1)
FROM golang:"${GO_VERSION}-alpine" AS build
# Retrieve source code
WORKDIR /app
COPY . /app
# Build application
RUN go build -X "GoVersion=${GO_VERSION}" -o ./webapp

FROM ubuntu:20.04 AS test (2)
ARG GO_VERSION=1.15.11 (1)
ENV GO_VERSION=${GO_VERSION}
RUN ./run_tests.sh

FROM ubuntu:20.04 AS run (2)
COPY --from=builder /app/webapp /usr/local/bin/webapp

Even if functionally it works, this creates unnecessary duplication in the Dockerfile.

Solution you'd like

There should be a way to avoid setting values to arg lines using the default.

Preferred output would be

### File: Dockerfile.webapp
ARG GO_VERSION=1.15.11 (1)
FROM golang:"${GO_VERSION}-alpine" AS build
# Retrieve source code
WORKDIR /app
COPY . /app
# Build application
RUN go build -X "GoVersion=${GO_VERSION}" -o ./webapp

FROM ubuntu:20.04 AS test (2)
ARG GO_VERSION
ENV GO_VERSION=${GO_VERSION}
RUN ./run_tests.sh

FROM ubuntu:20.04 AS run (2)
COPY --from=builder /app/webapp /usr/local/bin/webapp

Alternatives you've considered

For example, the given input

spec:
      file: Dockerfile.webapp
      instruction:
        keyword: "ARG"
        matcher: "GO_VERSION"
        ignoreDefaultValue: true

would change the matching logic to

# Matches
ARG GO_VERSION=0.1.0
arg GO_VERSION=0.1.0

## Does NOT matches
ARG GO_VERSION
ARG GO_VERSION=
ARG GOLANG_VERSION
ARG RUST_VERSION=GO_VERSION
ARG go_version

Anything else?

No response